Chasing down a brake issue

[HawkeyeScott]

I am looking for guidance on the brake system for my 1997 GMC Yukon. The brakes work well enough, but they work far better if I pump the pedal. For instance, if I approach a red light and push the brake just once, it will stop, but the pedal is pretty soft. If I push the pedal, quickly let off and then reapply, the brake is firmer. If I do that a second time, it gets very firm. Also, when I apply the pedal, there is a noticeable hissing/air leaking sound.

When I bought it in August, I machined the front rotors and put on new pads. I then bled the brakes at all four wheels and replaced the brake fluid. That didn't change much, if anything. I also adjusted the rear drums and they seem good to go. So I spent $28 and replaced what appeared to be the original master cylinder with a new Dorman one. I bench bled the new MC and then re-bled the truck at each of the four wheels. Unfortunately, the pedal feel hasn't changed.

At this point, I'm thinking it's the booster. My question, though, before I throw a fairly pricey part at is this: Would a failing booster produce the hiss and soft pedal (that can improve with pumping)? If it is a booster, would it make any sense to try and find a salvage yard special? Based on the age of anything sitting in a junkyard, though, that may not be wise.

Thanks.

 

[spasm3]

Usually the brake pedal us very firm the the brake booster does not work. But, the hissing sound makes me think the booster is on the way out.

When the truck is sitting still, does the brake pedal move lower under your foot? If its a dorman or cheap aftermarket, i wonder if its the correct master, since you have to pump it, unless its just bad.

 

[14Accent]

It's not the booster. A failed booster will give you a hard pedal, but no excessive travel. Those trucks always make a "hiss" sound when the brake is depressed.

How did you adjust the rear shoes? Through the access hole, or by removing the drum? If it was the latter, you need to use the access hole with the drum installed. The only way I've EVER been able to get a decent pedal out of these trucks with the old Kelsey-Hayes ABS system was to adjust the snot out of the rear brakes. You won't burn them up, but you will get a pedal. I'd also suggest replacing the brake hoses, after this much time there's bound to be some expansion under pressure than can cause poor pedal feel.

Are the calipers original, as well? They could probably stand to be replaced at this point, and I'm pretty sure caliper's for that truck are dirt cheap.

 

[HawkeyeScott]

I've read that about the booster and a hard pedal. Knowing the hiss is typical helps a lot.

I took the drum off when I adjusted the rear brakes. I adjusted until they barely would turn by hand, then quit.

 

[tiger862]

I would replace the booster. Only way I can explain it is when brake fluid gets into booster it swells seals and not only causes air leaks it also allows booster to bypass pedal assistance therefore goes to the floor. With most booster problems it is a hard pedal but I have seen several bad boosters that went to the floor. Only way to diagnose is replacement.

 

[andyd]

Spend 10$ for a complete new set of rear brake hardware Hoses are a good suggestion too. as well as hardlines. My brother recently bought a complete set from a dlr for about 100 for his '98

 

[14Accent]

Originally Posted by tiger862
I would replace the booster. Only way I can explain it is when brake fluid gets into booster it swells seals and not only causes air leaks it also allows booster to bypass pedal assistance therefore goes to the floor. With most booster problems it is a hard pedal but I have seen several bad boosters that went to the floor. Only way to diagnose is replacement.


Again, a bad booster can NOT cause a pedal to go to the floor. There's a direct mechanical link between the brake pedal and the master cylinder, through the booster. Unless the booster exploded inside, it wouldn't matter at all whether it was "functioning" or not. The only things that cause a low pedal are a fluid leak, air in the system, or poor adjustment/sticking parts. Keep in mind, "fluid leak" can mean an internal leak inside the master cylinder. I've ran into that a time or two.

I'll say again, start by jacking the rear end up and adjusting the rear brakes with the wheels installed. You may think you got them tight, when spinning the drum by hand, but I promise you can get a few more clicks out of it with the wheels installed.

 

[clinebarger]

Just to add to 14Accent's posts which I 100% agree with.....

If the drums are oversized....You may never get the shoes to have full contact with the drum!

The 10" Non-Servo style is JUNK on top of it.....Conversion to a 11" Duo-Servo setup isn't that difficult using backing plates off a Suburban. Some Tahoe/Yukon's already have 11", You may want to confirm this?

 

[NYEngineer]

In 1999 I bought a 1998 Yukon with approximately 50,000 miles on it. The brakes were awful. I immediately checked everything and ended up doing a full rebuild on the rears. New drums, springs and assorted hardware, shoes, wheel cylinders and the soft line that connects the rear to the chassis. Brakes were good after that.
I also learned that the bolts the front calipers slide on are garbage and fairly expensive. When I would do the front brakes, it was more cost effective to buy a reman caliper for 14 bucks as it came with new hardware. The hardware by itself was 11.
Don't cut the rotors on those either. If they need surfacing, just replace em. They're cheap enough. No one's left that does a nice job anyway.

Here's something you can try. We had an ongoing issue with my dad's 1988 Bronco when that truck was new. The truck had horrible brakes. It had been back to Ford a bunch of times and every time they said everything was normal.
Once, he was towing a trailer and went right through a red light with the front brakes locked up. We jacked up the rear and ran the rear wheels up to 30 mph. He stepped on the brakes and they stopped but it took longer than it should have. Then we ran it up to 55 and when he stepped on the brakes it was like he did nothing and then they finally stopped.
Between me, my dad, my brother and one of our employees, we were all usually pretty good and solving offbeat mechanical issues. We came up with this...
We removed the rear wheels and drums and watched to see how much movement we got with X amount of pedal travel. Turns out, very little. We removed the restrictor from the port on the master cylinder to the rear brake line. That helped immensely so we figured we were on the right track. While I was trying to cross reference wheel cylinders with smaller pistons in them in an effort to gain more travel quickly, our employee, George, who had a super gas dragster suggested putting a ten pound residual valve in the line.
We plumbed it in and checked travel. Worked like a charm. My dad ended up putting 325,000 miles on that truck without any brake issues ever again.
They also make residual valves in 2 psi in case you might try something like this. We went for the ten because that Bronco's rear brake springs were very strong so we felt there was a lot to overcome.

I also agree. It isn't your booster. They hiss.

 

[slybunda]

i thought long brake pedal travel ment air in the system or an issue with the front brakes?
wasnt aware rear brakes could make so much difference to pedal travel. could it also be caused by caliper pistons that are over retracting? if that is even possible?

 

[Lubener]

After your engine has been shut off for ten minutes, see if you get 3-5 power assisted pumps of the brake pedal before it goes hard. If it doesn't, you have either a bad booster or check valve. A booster will cause the pedal to go hard with more quick brake applications. Sounds like your master cylinder is going bad if pumping makes the pedal better.

 

[HawkeyeScott]

Thanks for the suggestions. I should have some time this weekend to do more adjusting on the drums. I'll start there and see if that doesn't do the trick.

 

[Donald]

New Dormant master cylinder. Why a crappy Dormant?