Soft Brake Help for a Newbie

[bivey77]

Hey everyone, This is my first time posting here, but I've been a lurker for about 6 months now. I inherited a 1990 GMC K1500 a few years ago that had thrown a rod in 2007 and sat in CA until it was towed to Denver in 2014. I just got around to fixing it March-May of this year (had to replace the engine...cracked block). This forum has definitely helped me get the car running. Thanks for all your help!

Anyway, about a month ago I noticed a puddle of brake fluid at my driver's side rear wheel. It was the day before I was going on vacation, so I ignored the problem until I got back, about 10 days later. When I started working on it, the portion of the master cylinder to the rear wheels was completely dry. The truck was parked on the street, so I filled up the master cylinder, started it up, and carefully backed it into my garage (ps probably the most terrifying thing I've done, backing up a truck with my wife behind it guiding me into the garage and suddenly feeling like I have zero brakes).

I got to work on the truck, replaced the wheel cylinders, brake shoes, all springs and had the drums resurfaced at my local NAPA. I adjusted (duh) the adjusters until I could barely get the drum on and there was a little resistance when I spun the wheel. I put everything back together, bled all 4 brakes, and that's when the real problems started.

I have bled the brakes about 6 times, tried both the one-person method with the tube and a bottle, and I've tried the 2-person method with my wife pushing down the brake, me opening/closing the bleeder valve, her letting off the brake and repeated until no bubbles came out of each brake. I also did it in the right order, RR, LR, RF, LF. But I still get a soft pedal when I start the car.

I even replaced the master cylinder, thinking I had somehow screwed that up. I bought a new one from summit racing, bench bled it with the syringe until no bubbles came out of either portion of the master cylinder.

And yesterday I tried bypassing the RWAL per another post in this forum. No change

No matter what I try, I still have the same problem: brakes feel great when the engine isn't running, but as soon as I start the vehicle, the pedal is as soft as can be.

Does anybody have any ideas of what I'm missing?

Thanks for your help!

 

[east302]

Was it this way before you did the rear work? That is, have you ever known it to have good brakes?

If it was bad before, I'd look at the rubber lines on the front. Sometimes they can expand a little as they age, but there's also that rubber segment on the rear circuit at the axle.

One way to test it (but at a real risk of ruining your hoses) is to clamp off the rubber hoses, check the pedal. If it's good, remove the clamps one at a time until the pedal is soft. That will be the wheel with the issue.

Does pumping the brake help firm it up?


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[bivey77]

Thanks for your reply. The brakes worked great before I did the work on the rear wheels. It's entirely possible that the rubber lines are shot, but it's strange that they would go bad right at the same time as the wheel cylinders. I'll check them out though and get back.

Pumping the brakes makes them firm up when the engine isn't running, but when it's started pumping does nothing.

 

[east302]

Drums could be too far out since you only had them resurfaced. Did the shop comment on how close they were to discard thickness (maximum diameter)?

Since they don't firm up while pumping when braking (air in the lines) it kind of sounds like an adjustment issue.

I have a 98 (has different ABS system but otherwise the same) and went through a similar issue. After three master cylinders, three sets of drums, several rotors and pads, new drum backing plates, brake lines and ABS unit, I gave up.

A year later the booster went and after replacing it the pedal was great. I have no idea why that resolved it. Maybe something with the plunger at the master cylinder.




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[bivey77]

The drums had a max diameter on them and the shop told me they had enough material to resurface. I didn't verify the diameter, though. When I set the adjusters, I made sure there was a bit of resistance.

I'm starting to wonder if it's the brake booster... honestly I don't have any experience with it.

 

[east302]

Try giving the auto adjusters a chance to set the shoes. It has new shoes, adjusters and springs/hardware, right? A healthy dose of grease on the adjuster is worthwhile so that it can spin easily.

There are two types of drum systems, leading-trailing and duo-servo. Leading-trailing is RPO code JB5 and the other maybe JB6 (check the RPO label in the glovebox). I forget which one is which, but one auto adjusts when backing up and braking hard while the other adjusts every time the parking brake is used. Neither do a spectacular job, but you can get a good pedal out of these trucks.

Try doing the hard braking in reverse several times, see if it firms up.

Also cycle the parking brake several times. In park, fully press the brake pedal and parking brake simultaneously. I've heard where some have done it while driving slowly and gotten a better pedal--presumably because it adjusted in a good bit. It's worth a shot.

You'll know when the booster is bad. The pedal is firm with reduced braking ability.


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[Dano214]

I’m chasing the same problem with my 97 same exact issue, Pedal is solid as a rock then I start it and it’s soft and squishy. I’ve replaced the master and the booster, pads and shoes. I’ve bleed it so many times I should have bought a 5 gallon pail of brake fluid and still the same issue. It’s very frustrating, I just got some stainless braided lines to replace the old rubber ones, I’ve done the back and didn’t notice any chance, hopefully I’ll get the front done this weekend. I don’t have a clue what else to try.

 

[bivey77]

Sorry it took so long for me to respond. I'm still trying to figure this out and haven't had any luck.

Last weekend I had a friend come over that has much more experience with brakes than I do. He helped me bleed the brakes again. We started with the pass rear and he said that while that one was bleeding he had never seen so much air in the brakes. We finished up, and started the car. The Pedal went all the way to the floor again. We put the rear end on blocks and took the wheels off and checked the springs/drum/shoes. Everything looks great. And while it was up, I engaged the parking brake and then we tried to rotate the wheels, but we couldn't. I released the parking brake and tried it again with just the brake pedal. Again, we couldn't rotate the wheels. So the shoes are engaging the drum, but when I start the car, I still have no brakes.

So, we started looking for leaks. The only place we could find was at the gasket at the master cylinder between the reservoir and the cylinder. There wasn't a lot, but you could see that the gasket was wet. We dried it off and pumped the brakes a few times and saw the gasket was a little wet again. So, I ordered a new one (again). I'm not convinced this is the problem though.

Do I need to do anything with the combination valve before/during/after bleeding the brakes?

Honestly, I haven't tried the recommendations to back up and brake. I'm nervous driving it since I feel like there is no braking power at all and I live in a pretty congested neighborhood (lost of cars parked on the street).

 

[east302]

I thought you just had a soft pedal...didn't realize the pedal was going all the way down like that.

If you're getting fluid from the cylinders and calipers, then the combination valve hasn't tripped. I'm not familiar with the bleed procedure (if any) for the earlier combination valves. There may be black rubber caps covering pins that extend when the brake pedal is pushed. On the later (96?+) you would keep the pin from extending while pumping the brakes to get the valve to recenter. Not sure about bleeding, though.

Swap the master cylinder if it's leaking. Before connecting the lines (but with it installed on the booster) keep the outlet ports plugged, crank it and see how the pedal feels. Reconnect and bleed one circuit at a time and check the pedal. That could, at least, tell you if it is the front or rear circuit that is causing the issue...assuming that the MC replacement doesn't solve the problem.


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[kennythewelder]

The MC gasket, should not be wet at all. You have a leaking MC. Any leak lets air in, and will give a soft peddle. I did the GMT 800 MC swap on my 97 about a year ago. I bleed my brakes 6 times each @ each wheel, filling the MC after the rear bleeds, and again after the front bleeds. The I repeated the process again. So I bleed each wheel 12 times, and before I installed the MC, I bench bleed it. My brakes are amazing, and the fluid is crystal clear.