NBS master cylinder Swap

[thegawd]

hydroboost right? that's scary!

Ok, so I did have this happen to me. I was taking about 3,000 lbs of busted concrete to the landfill on my '00 CCLB K3500 SRW. When I left my house I heard a little unusual whine, but didn't think much about it. I took off and about 1 mile on the stop light the brake pedal went to the floor. I took off and a few hundred feet later I felt the steering wheel hard. I kept going another mile and made a left turn (then realized both steering and braking where gone) and got out to check under the hood, but didn't see anything broken.

Without brakes or steering wheel assist, I contemplated calling someone and getting truck towed. Luckily, it was about time for schools to get out (20 MPH zone I had just passed,) so I decided to limp back home. I got back on the main road and was on a slight decline, so I rode on the brakes and used the ebrake at the stoplight. I didn't think about downshifting, but was going about 25-30 MPH. That probably would of helped me. I finally did make it home, but it was a little scary.

So what happened is the steering pump shaft had broken and the pulley was just spinning there. I think I read on a different forum that this has happened before, can't remember if common or not.

 

[thegawd]

are you saying that you had a vacume booster fail and your pedal was still hard and able to function properly just hard as hell like the oldschool unassisted brake system?

I had a ruptured line happen at the same time so maybe that why mine was different.... IDK

Al

Not on mine!
Shunting on the driveway (very briefly as no cooling) without the serp belt I thought my difficulties holding it back were a combination of high(ish) idle speed and lack of drag (water pump, alternator, PAS pump) and maybe a bit of corrosion on the rotors. Only later did I learn about Hydroboost. Pedal was at the top of its travel at all times and rock solid.[/QUOTE

 

[stutaeng]

are you saying that you had a vacume booster fail and your pedal was still hard and able to function properly just had as hell like an oldschool unassisted brake system?

I had a ruptured line happen at the same time so maybe that why mine was different.... IDK

Al

Hydroboost assist from power steering pump was gone (or so it seemed.) In retrospect, I think I did still have braking power, but because I was loaded it seemed like I didn't.

P.S. I DIDN'T have to change diapers when I got home, LOL. But did go inside and told my wife I almost pottied on myself!

 

[Schurkey]

so with a failed booster you will still have brakes but it seems to come at the end of the peddle travel.

A BOOSTER failure results in a high, hard pedal, poor braking power. Pedal effort is higher than a manual brake system, because there's generally less leverage from brake pedal to booster, you're still moving the dead-weight guts of the booster, and the hydraulic system of a boosted application may have less hydraulic advantage than a system intended for manual brakes.

A HYDRAULIC failure in the brake fluid system (I don't mean the hydroboost system) results in a low pedal, poor braking power. With a single-circuit master cylinder, there might be "no" braking power. A dual-circuit master (mandatory starting in '67) you should have two or three wheels with at least partial brakes. Typically, either the front or the rear. Some vehicles have one front, one rear on a diagonal--left front/right rear, right front/left rear. Volvo and others used two-piston front calipers, so one hydraulic circuit worked a piston on each caliper, plus a rear brake. The other circuit worked the other piston on each caliper, plus the other rear brake. A failure in either circuit still left you with both front, and one rear brake.

A failed booster AND a failed brake fluid hydraulic circuit will get you double-****** braking power at the end of a non-assisted brake pedal stroke.

 

[thegawd]

thanks for clearing up my mistake Shurkey. thats an experience I dont want to go through again.

 

[Pinger]

are you saying that you had a vacume booster fail and your pedal was still hard and able to function properly just had as hell like an oldschool unassisted brake system?

I had a ruptured line happen at the same time so maybe that why mine was different.... IDK

Al

Hydroboost not vacuum (serp belt not fitted) and yes, solid brake pedal like an oldschool unassisted brake system.

 

[stutaeng]

you know I had vacume leak symptoms prior to this failure but I had no idea. it wasnt until I watched a really good video from a mechanic in Alaska where he went through the entire failed brake booster troubleshooting and replacement before I realized.

generally when a vacume booster fails, the diaphram around the plunger tares or disintegrates in some way and the you can hear a massive vacume leak in the cab or none at all because it's a very small leak.

prior to this massive audible leak it was all ready leaking I just had no clue. the symptoms were related to rich fuel trims. I'd reset the code and it wouldnt return for a day or so.

so the guy in Alaska puts a scammer on the truck and loads up the short term and long term fuel trims. he presses and holds the brakes down. if you have a failed booster one of those fuel trims will rise all the way up to 100% if you hold it long enough. then when he let off the brakes and gave it fuel the truck almost died, it fell on its face.

so prior to hearing a vacume leak that's is how you can troubleshoot a failed vacume booster.

here is that video...

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up until this happened I was collecting the parts to do the MC "upgrade." I had the line adapters, still do, and free access to a 2005 Tahoe, hahaha I also still do. but I wanted to know more about the hydraulic systems, how they work, and I truly believed that the individual parts from each brake system has to be rpo specific, (I only order pads/drums/discs/wheel cylinders/brake calipers by RPO code, a lesson learned by only having one vehicle, getting the wrong parts but not realizing it until I had it all torn apart!) so I reached out for an explanation and I got a very detailed explanation. And since I all ready agreed with the explanation that closed the books on the mc swap on my Burb indefinitely. It has JB7 brakes and so I replaced only what was broken, a booster and the line entirely for the front right passanger as it was the last one on my truck that wasnt replaced by me back when I first got the truck. what I could see if that line was perfect, it was what was hidden along the frame under the plastic shroud that was rotten, I dont even know how it hadn't ruptured prior during emergency stops. prior to the failed booster I had no complaints about my Burbs brakes. they worked flawlessly and so does the abs system which I regularly exercise for fun.

I have more brake parts for the burb, all new all 4 corners when though it dosent really need it, it's been years since I replaced what needed to be back when I got it other than a few sets of pads.

Now my Sierra has the JB5 brake system............... but it was replaces entirely absolutely everything except the MC/Booster because they were not broken.... my buddy owned this truck and he fixed a tonne of ****, the right way.

Al

That was interesting with the fuel trims going really high. So I guess the diaphragm inside the brake booster is ruptured, causing unmetered air into it the engine? And ECU just starts dumping more fuel, based on O2 sensor readings...

 

[Zach Ondash]

So I’ve got a 1994 Chevy K1500 extended cab. Previously my brakes have majorly sucked. Super spongey, not even enough power to lock up a wheel, and my input pressure on the pedal is not proportionate to the braking power. I went ahead and got an nbs master and swapped it on. Pedal travel is less and it’s firm but it feels like I have no power brakes. Pushing absolutely as hard as I can results in a slightly above normal stop(normal being how you would generally slow down for a stop sign or something). I believe the factory bore for the obs is 1.25” and the nbs master I got is 1.34”(although they did a poor job of noting that in the description). Would a 0.09 increase in bore diameter really make my braking pressure that much less or is it possible I have a booster issue?

 

[Schurkey]

Your original master diameter was probably 1.125, with a 40mm quick take-up bore at the rear. The stupid "NBS" master cylinder "upgrade" is a terrible mistake on any vehicle with low-drag front calipers like 1500s and most 2500s. (JN3, JB/JD5, JB/JD6 brakes)

Verify the front calipers slide nicely on the pins, and the pistons aren't stiff in the bores which is ultra-common on older calipers. The pistons don't have to be completely seized to give problems.

Get rid of that improper master cylinder, get the RIGHT one for your vehicle. Bench-bleed the new master so there is NO air in it, (which seems to take three times as long as people expect) and then bleed the brakes properly using a scan tool. When the brakes are bled, make a point of adjusting the hateful leading/trailing shoe rear brakes, and then use the park brake regularly to keep them adjusted. Be sure the park brake cables aren't over-tightened.

MAYBE there's some defect in the booster; or some other hidden problem in the ABS, or a plugged rear brake hose, etc.

Most likely, air in the ABS and it's never been bled properly with a scan tool; combined with dreadfully out-of-adjustment rear brake shoes.

(A few 1500 pickups have decent, Duo-Servo rearbrakes; and I guess the Tahoe/Suburban and maybe vans have Duo-Servo; but most 1500 pickups have the crappy leading/trailing shoe, 254mm (10") rear drums.)

 

[Zach Ondash]

Your original master diameter was probably 1.125, with a 40mm quick take-up bore at the rear. The stupid "NBS" master cylinder "upgrade" is a terrible mistake on any vehicle with low-drag front calipers like 1500s and most 2500s. (JN3, JB/JD5, JB/JD6 brakes)

Verify the front calipers slide nicely on the pins, and the pistons aren't stiff in the bores which is ultra-common on older calipers.

Get rid of that improper master cylinder, get the RIGHT one for your vehicle. Bench-bleed the new master so there is NO air in it, (which seems to take three times as long as people expect) and then bleed the brakes properly using a scan tool. When the brakes are bled, make a point of adjusting the hateful leading/trailing shoe rear brakes, and then use the park brake regularly to keep them adjusted. Be sure the park brake cables aren't over-tightened.

MAYBE there's some defect in the booster; or some other hidden problem in the ABS, or a plugged rear brake hose, etc.

Most likely, air in the ABS and it's never been bled properly with a scan tool; combined with dreadfully out-of-adjustment rear brake shoes..

How can I determine the factory bore diameter as well as the brake code(ie; jb5, jb6, etc.)