NBS master cylinder Swap

[L31MaxExpress]

IF (big IF) there are no external leaks, and IF you are not applying more pedal force once stopped...a slowly sinking pedal is classic symptom of an internally-leaking master cylinder. In other words, you need a new master cylinder that works properly.

If you are increasing pressure on the brake pedal once stopped, having the pedal sink a little bit and then hold firm is totally normal, for all the reasons given in a previous post--caliper flex, hose ballooning, seal distortion, heck, even firewall flex can make a difference in perceived pedal height. If the pedal continues to sink...the master has failed.



I guess.

I can maybe apply increased pedal pressure. YOU can perhaps apply increased pedal pressure. The Original Poster, and everyone else who's ever completed this "mod" can, perhaps, apply increased pedal pressure.

They (we) shouldn't have to. And my knees aren't what they used to be.

Can their sister, mother, teenage kid, valet parking attendant, or the NEXT OWNER of the truck, apply that much pedal pressure? At least one guy removed his GMT800 big-bore master cylinder because he found that in a panic stop, he could NOT apply enough pedal pressure unless he used both feet and a squirt of adrenaline.

The GMT800 big-bore master cylinder "mod" is NOT SAFE particularly if installed on ****** little JB3*-style power boosters. It's bad enough on the larger JB5--6 boosters. Folks are using the GMT800 big-bore master cylinder to crutch another problem in their vehicle's brake system. Essentially, they're fixing the wrong thing. They should be adjusting the rear brakes, removing air from the system--abs valves, or whereever--or replacing a failed step-bore master cylinder with a properly-functioning step-bore master cylinder.

*Note that the JB3 brakes were so ****** that GM quit using them in the early-'90s. My 88 K1500 came with JB3 brakes, I've since converted to JB6--but the big change was removing the "10-inch" drums for the 11.15" duo-servo drums.

Here's the JB3 booster compared to a JB5--6 booster:
Photo 1. JB5 booster (left) and JB3 booster (right).
http://hbassociates.us/K1500_2020_Brake_Booster_02.jpg

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So far as I know, the ONLY leading/trailing shoe rear brakes on GMT400 series vehicles are the "10-inch" drum units. Officially, they're 254mm, and the drums have a metric measurement cast into them. They're only used on JB 3 and JB/JD 5 brake systems. From JB/JD 6 and higher, you get Duo-Servo drums of increasing diameter and width, or rear discs.

I can speak 100% from experience. If you have fixed everything and want brakes that actually work better it is a hydroboost and matching correct larger step bore master cylinder swap away. Then use the Police/Limited Tahoe aka 8800 GVW 3.2" front calipers in place of the 2.9" front calipers on the wider front rotors. The duo servo rear brakes are also a nice upgrade. Even my 5-lug G20 van had the duo servo rear drums. The old G20 stopped great until I put enough cam in the thing to hurt the idle vacuum. Hydroboost swapped it about 15 years ago too.

Friend of mine built his 68 Chevelle with a big cammed 5.3. It had very little vacuum boost and a rock hard pedal. Asked me what to do about the booster and after taking my Tahoe around the block, ordered a hydroboost setup for the car. With hydroboost and 4 wheel discs with an adjustable proportioning valve adjusted to match the car that Chevelle stops on a dime.

 

[L31MaxExpress]

Absolutely no external leaks - confirmed by fluid level in reservoir not budging.
Probability of a new (Raybestos) MC being bad out of the box?



I'll pay more attention to just how much pressure I'm applying.
If I just sit there at a halt with the same pressure on the pedal as it took to bring me to a stop the pedal remains where it is. More pressure and it starts sinking. Steadily and slowly it sinks. Solid doesn't happen.
Probability of a new (Raybestos) MC being bad out of the box?

The factory master cylinder on my Express van was replaced by GM under warranty when the van was less than 2 years old and had less than 15K miles on it. The pedal would do exactly what you are describing, then occasionally sink straight to the floor. I remember riding over to the dealership after they could not duplicate it and having a technician go on an extended road test until after a stop it suddenly just sank to the floor.

 

[Pinger]

I just tried it sitting on the driveway without the engine running and the pedal sinks - just takes a lot more force on the pedal. When the last MC failed it's failure was preceded by this type of behaviour.

Do I just contact Rock Auto and ask for a replacement?

 

[L31MaxExpress]

One further thing I have noticed that makes a big difference in stopping power. Drilled and slotted rotors run cooler and are much less prone to warping under hard use.

Ducting cooling air to the rotor via scoops and ducting also help the rotors stay cooler. I used cutouts in the lower valance to duct air to the brake shields on the Express van. Really reduced the rotor temps when having to use the brakes coming down a mountain with a trailer. Combined with the wider 01-02 G3500 front rotors and matching larger rear drums on the 10.5" I run now the front brakes operate way cooler than it did with the 1500 brakes. The 1-ton running gear swap was worth it for the improved braking alone not to mention the much stronger full float rear end.

 

[Schurkey]

Probability of a new (Raybestos) MC being bad out of the box?

The new "AC-Delco Professional" master cylinder that went onto my '88 was made in Communist China. I looked all over for a Non-Chinese master cylinder, and had no confidence in finding one.

I d say a faulty "new" master cylinder is very likely. NOBODY sources product from Communist China to improve quality. They source from China to improve profit margins.

Yes, the next phase of that problem is randomly and unpredictably having the pedal drop like a paralyzed falcon.

I'll pay more attention to just how much pressure I'm applying.
If I just sit there at a halt with the same pressure on the pedal as it took to bring me to a stop the pedal remains where it is. More pressure and it starts sinking. Steadily and slowly it sinks. Solid doesn't happen.

Give the master cylinder a few "Sixty-Second Tests".

Get in the vehicle, start the engine so the power booster is working.

Press the brake pedal your "usual" amount--typical force for coming to a normal stop. HOLD pressure for sixty seconds, while feeling for pedal drop. There should be ZERO pedal drop. Release the brake pedal for a while.

Repeat the test with additional pressure on the pedal--like coming to a quick stop. Again, HOLD that pressure for 60 seconds. There should be ZERO pedal drop. Release the brake pedal for awhile.

Again--stamp the pedal like you're coming to an EMERGENCY stop. HOLD for sixty seconds. ZERO pedal drop allowed.

If you hold steady pressure, and the pedal sinks...master is faulty or you have external leaks.

I can speak 100% from experience. If you have fixed everything and want brakes that actually work better it is a hydroboost swap matching correct larger step bore master cylinder swap away.

I'm a Hydroboost newbie. The single vehicle I have experience with Hydroboost on, is my '97 K2500. Truck has ~160K miles on it.

The brakes are powerful enough, and have low pedal effort--but the pedal "feel" sucks. I've heard that complaint from some others, but not from everyone who has Hydroboost. I don't know if it's an inherent problem, or if it just affects some vehicles as a symptom of something that isn't right.

Note that I have only put about one thousand miles on this truck, maybe less, and I've never had the rear drums off to inspect the shoes. The first ~200 miles, the PS cooler was hemorrhaging fluid, and that couldn't have done the Hydroboost any favors.

 

[L31MaxExpress]

The new "AC-Delco Professional" master cylinder that went onto my '88 was made in Communist China. I looked all over for a Non-Chinese master cylinder, and had no confidence in finding one.

I d say a faulty "new" master cylinder is very likely. NOBODY sources product from Communist China to improve quality. They source from China to improve profit margins.

Yes, the next phase of that problem is randomly and unpredictably having the pedal drop like a paralyzed falcon.


Give the master cylinder a few "Sixty-Second Tests".

Get in the vehicle, start the engine so the power booster is working.

Press the brake pedal your "usual" amount--typical force for coming to a normal stop. HOLD pressure for sixty seconds, while feeling for pedal drop. There should be ZERO pedal drop. Release the brake pedal for a while.

Repeat the test with additional pressure on the pedal--like coming to a quick stop. Again, HOLD that pressure for 60 seconds. There should be ZERO pedal drop. Release the brake pedal for awhile.

Again--stamp the pedal like you're coming to an EMERGENCY stop. HOLD for sixty seconds. ZERO pedal drop allowed.

If you hold steady pressure, and the pedal sinks...master is faulty or you have external leaks.


I'm a Hydroboost newbie. The single vehicle I have experience with Hydroboost on, is my '97 K2500. Truck has ~160K miles on it.

The brakes are powerful enough, and have low pedal effort--but the pedal "feel" sucks. I've heard that complaint from some others, but not from everyone who has Hydroboost. I don't know if it's an inherent problem, or if it just affects some vehicles as a symptom of something that isn't right.

Note that I have only put about one thousand miles on this truck, maybe less, and I've never had the rear drums off to inspect the shoes. The first ~200 miles, the PS cooler was hemorrhaging fluid, and that couldn't have done the Hydroboost any favors.

Not sure what does not feel right to you about it to answer properly. Having swapped 3 different vehicles I own the hydroboost pedal does feel a bit "loose" if you compare it to a normal vacuum boosted setup. That being said it has dramatically more assist from the booster and the pedal effort is much less for a given brake application. First vehicle I ever drove with hydroboost was a mid 90s AWD Astro van. Pedal felt pretty limp in it but the brakes worked smoothly and quickly with only moderate pressure and that thing would stop on a dime if you really got down on the brakes hard.

 

[Knuckle Dragger]

As I said before, took all 275 lbs of me standing on the pedal with both feet to get that pressure. A person cannot even get the 1.13" bore master to allow the pedal to touch the cab floor if it is working correctly.

Well that would be your experience. It doesn't make it an absolute. I completely understand and respect where you are coming from. There most definitely needs to be a cautionary tale on changing anything on the vehicle outside of what the manufacture designed, especially brakes. If joeblow with zero or near zero mechanical expertise or experience starts bolting parts on based on what they read on the internet it could be catastrophic. Same thing applies if people don't realize there are trade offs for every modification. I think we can have reasonable discussions on the pros and cons without calling each other names or implying mental defects because we disagree. Look what that mentality has brought us to in politics and such. Surely car guys can do better.

 

[Schurkey]

I'll pay more attention to just how much pressure I'm applying.
If I just sit there at a halt with the same pressure on the pedal as it took to bring me to a stop the pedal remains where it is.

Ducting cooling air to the rotor via scoops and ducting also help the rotors stay cooler. I used cutouts in the lower valance to duct air to the brake shields on the Express van. Really reduced the rotor temps when having to use the brakes coming down a mountain with a trailer.

Most everyone uses their brakes incorrectly.

OK, that's a bold statement, and it does matter what your priorities are. When I transported my elderly and infirm parents and in-laws, I used my brakes incorrectly, too. Their comfort meant more to me than brake life. Obviously, when coming down the mountain with a trailer load, or in an emergency, or driving on wet/icy/gravel roads, you don't have a lot of options--you're going to brake however you need to to get stopped.

Ideally, when there is a choice, to maximize brake life, the faster you go, the harder you stop. As your speed reduces, particularly below 15--20 mph, you begin releasing pedal pressure. As the vehicle comes to a stop, you're releasing even more pressure. The suspension returns to near-normal height in front just as the vehicle stops.

While stopped, you're holding only as much pressure as it takes to remain stopped--you're not hard on the pedal.

WHY:

• The rotors are vented. Drums usually have fins cast into them. Rotors and drums are designed to get hot, They're also designed to move cooling air.
• The pads/shoes are insulators in addition to being friction material
  
• Brake fluid (especially old fluid) expands when hot, potentially boiling if it's absorbed water.
• Most brake fluids LOVE to absorb water
• The most-contaminated fluid is usually in the wheel cylinders/calipers.
• Wheel cylinders including calipers have rubber parts in them, wheel cylinders do better if they don't get overly-warm.

SO:
Vented rotors are essentially centrifugal air pumps. The faster they spin, the more cooling air they move. At low speed, they don't move much air. Braking hard at high speed creates heat, but the rotor is turning fast and pumping lots of cooling air. The rotor gets hot, which is fine. It's designed to get hot. The cooling air carries away the heat, and everything is good.

The pads get hot on the surface that contacts the rotor. Pads are designed to take some heat. "Street" compound friction material works good when cold, better when warm, and not so good when really hot. "Racing" pad material works better when hot, but doesn't really apply to street-driven vehicles.

If the pads stay in contact with a hot rotor, eventually they get hot all the way through, and then they transfer heat to the caliper.

The caliper transfers heat to the fluid. Bad things happen then.

Use the brakes harder when there's more cooling airflow. DON'T glide to a stop with the brakes applied gently for a long time; that gives heat time to travel through the brake pad and warm the caliper and fluid.

And don't clamp the pads tighter than needed to the hot rotor when stopped, for the same reason. HOT friction material can actually transfer to the HOT iron--rotor or drum. Then you have brake pulsing until it wears off again.

Get ON the brakes, and then get OFF the brakes. The point is to let the iron (rotor and drum) get hot while they're still turning fast enough to pump air, while minimizing the temperature and time-at-temperature of the pad, caliper, and fluid. It makes more difference with disc brakes, but it's worthwhile on drums, too.

Folks will ***** that you're trying to "throw them through the windshield" and "you're really hard on your brakes". In fact, your brake linings will last longer. Taking a long time, braking too gently, is what's hard on brake linings and brake fluid. You put a lot of heat into the rotor when it's turning too slow to pump cooling air; and you brake for a long time so that heat travels through the pad into the caliper and fluid.

 

[L31MaxExpress]

Has anyone else on here tried brake cooling ducts? Relatively cheap, simple and work really well especially for a tow vehicle. I have done the mod to both my Express van and my Tahoe. Allstar makes a metal duct attachment piece for both left and right hand side that you cut out part of the rotor backing plate to fit into place. I pop riveted them into place. Then bought the 3" hose in a 10' section. Then used two ABS scoops that I attached to the holes in the lower valance. Route the hoses and support them in 2-3 places with cheap metal 3" pipe hangers like you see sprinkler pipes hanging from in buildings. I made the modification in one afternoon while changing the rotors for about the 4th time. When I had the 1500 front rotors it cured my rotor warping issue I had towing despite having adequate trailer brakes and a properly adjusted controller. The large scoops facing into the wind at 40+ MPH forces alot of cooling air directly into the backside of the rotor where it can flow through the vanes and help cool it effectively. If someone felt the need they could use a pair of those 12 volt bilge blowers for a boat in-line wired to a relay connected to the brake switch and have rotor cooling while stopped.

 

[Erik the Awful]

Has anyone else on here tried brake cooling ducts?

No, but we kinda did the opposite in our race car. [storytime] During practice our modified spindles were causing crazy bumpsteer, so our team captain wisely decided to swap in stock spindles for the race. I went to put them on and the factory dust shields were still attached.

"We don't have time to remove them. You have to split the bearing to get them off. Just leave them on, they'll be fine. We're running high-temp fluid and race pads."

I had the first stint, did two hours in the car, pitted, and handed the car off to him. Thirty minutes in he called on the radio. "The brakes are acting funky, but I think I'm gonna ride it out."

Ten seconds later, "I HAVE NO BRAKES!"

He managed to keep the car on track and limped it into the pits. I cleared the bay for him and as he pulled in, there was green/blue/yellow fire coming out of the wheel. Our fire extinguisher was trackside in the hot pits with our fuel jugs, about two hundred yards away. I grabbed our pit neighbors' fire extinguisher, and quickly realized you can't put a brake fire out from the outside of the rotor. I couldn't fit my arm under the front of the car to get the nozzle in that way. I pulled the pins on the "frunk" (MR2, engine in back), and we had holes cut in the wheel well right next to the caliper. I got the fire out.

After everything cooled down we pulled it apart. The dust shields trapped so much heat that we melted completely through a set of full-bore race pads, through the backing plate (can't hear the grinding when you're racing), and halfway through the pistons. When the piston finally fell out of the seal, the brake fluid, which is flammable when it's over 900*, hit the screaming hot rotors and flambeed the calipers.

We had spare calipers, and a teammate jetted to town for a set of stock pads. I grabbed a hammer and chisel and had the dust shields off in ten seconds each. We ran the entire rest of the race on stock pads with no issue. Apparently two other teams set their brakes on fire that race. #texasproblems
[/storytime]