brake problem...

[Longhorn Man]

I got a call from GMC Junkie recently on her son't truck, and was unable to help.
69 I believe, 1/2 ton, 4 wheel drums.
His pedal was going to the floor, took it to a brake shop, and they replaced the master cylinder. They checked everything 2 or 3 times, to no avail... The problem is, no matter how much they bleed it, they are constantly getting air in the stream coming out.
Remember, this is a brake shop, and no matter how much we wanna laugh at the chain stores, lets admit it, for the best part, they know what they are doing. They were checking lines, looking for leaks, looking at the wheel cylinders, everything.
They ended up giving it back not working, and charging a cut rate (probably only for the master/labor to install).
So she had it towed to her regular mechanic. He has been turning wrenches for her for quite a while now, and sounds like he always does her right... he called her today and was saying the same thing.... can't figure it out, makes no sense.
Her son (junkie jr)is talkin crazy talk... like sellin his rig.
We can't let him do that. He's one of us... just real quiet.
Can anyone throw out some sugestions? Everything I mentioned, he already looked at, and like I said, her regular mechanic sounds like he knows his stuff... she never has anything to say bad about him.
We are talkin an all stock 69 or 70 1/2 ton chevy.... 4 wheel drums, unmodified for the best part, I am almost poisitve it has manual brakes (no booster).
Anyone have any ideas?
Jodie should be getting on here this evening and answering specifics that I didn't cover... she just wanted to get this up on here as soon as possible and she's at work.

He has had some damage due to a drunk driver, but it's been 6 months and driven most every day.

I don't think I see a boosterr in there... I am 99% positive it is manual brakes.

[El Jay]

Is the master cylinder new, or rebuilt?
I've gotten a bad rebuilt MC in the past.
Just an observation and my .02.

[steelhorse]

Alright it could be a bad master(especially if its rebuilt) after that plug the lines at the master and see if the petal if firm if so its not the master.Then check all the lines for cracks,a cracked flare at a wheel can suck air and not show a visible leak.Does it blow air at all the wheels or just the ft/rear try to narrow down the problem area.

[PanelDeland]

Was the MC bench bled?
Check lines at MC/Proportioning valve for tightness?
Air has to be getting in somewhere for it to bleed out.If nothing else was changed then the MC would be my guess.Maybe use a MC bleeder kit hoses from the outlets on the MC back up into the reservoir.They would need to be under the fluid level.Then pump the brakes and watch the reservoir.If the bubbles don't stop after a while the MC must be sucking air from somewhere.

[DREAMS OF A 67]

I would think it has to be the master cylinder I agree with El Jay,Ive gotten bad rebuilds too.

[68gmsee]

If they say they are absolutely sure the master cylinder (always a suspect) is okay, then as steelhorse mentioned, plugging lines is the only way your going to solve this one.

[Huck]

I don't believe they re-pop the original Master Cylinders so you are probably dealing with a universal one. The rod that engages the piston --ie from the brake pedal to the MC piston should have virtually no play. If there is free throw then you have a problem there. Next block off the output lines from the MC and check for a nice high, hard pedal. If not you know it's up stream. If you do and you continue to have a dropped pedal, you know it's down stream. Now just block off the front or rear till you find the problem. You should be leaking fluid if there is a hole in the lines or in a wheel cylinder.

[69_hafton]

Does the proportioning valve have a pin that needs to be held in while bleeding? I can't remember what the prop valve looks like that goes with 4 drum setup now.

[woodwright]

When I changed the MC on my gmc years ago it did the same thing. I bench bled it, then installed it and the pedal would just keep going to the floor. I finally ran out of time to mess with it so I moved it out of the way and it started working. All I can think of is there was a bubble in there somewhere and when I moved it around it finally came out.

[Boog]

It's a process of elimination. I assume it happened all at once.
1st thing is check and adjust all brake shoes. If they are way out of adjustment you will get a lot of pedal travel.
Now the mc has been replaced and the condition is the same. Use the mc bleeder tubes to separate the mc from the rest of the system and eliminate it as the culprit. If it comes up clean then the problem is down stream from there.
All three rubber hoses and wheel cylinders are suspect.
A loose nut on one of the hard line connections maybe.
You could borrow or rent a pressure bleeder to put some good pressure on it and maybe find a leak that way as well as getting ALL the air out of the system.
Tell Jr not to give up. It's not that bad.

[oldgold70c10]

I guess you checked the obvious things first: hoses not leaking, wheel cylinders not leaking, shoes adjusted correct, fluid level kept high in MC during bleeding (I didn't do this and spent a lot of xtra time trying to bleed until I looked).

If you mash the pedal real hard, does the brake light come on?

If all above checks OK, I'd say master cylinder is bad. They aren't expensive, I got one from NAPA, remaned for $20.

[truckdude239]

i would try another mc the one i put on my truck was bad off the get go

[GMCjunkie]

Thanks for the replies yall. I dont know a thing about the brakes, I know theyve checked everything under the sun, but, Im going to print this out and give it to Terry, my mechanic. its just really frustrating. Ill let yall know if we get it,
thanks again, and THANKS Andrew for posting for us!!

[truckdude239]

i stoped buying reman mcs for about 3-4 buxs more its cheaper and safer to get new parts. i sure the brake store put a reman mc on it probley the whole problem

[Pont406]

I had a leaking front wheel cylinders cooper washers once. Sounds very similar. I would bleed the system. Test drive it and crack open front bleeders and I could see air bubbles. Washers didn't leak fluid but were sucking air. I replaced washers and was able to bleed the system.

[chipflyer]

I went through the exact same thing about a year ago on my 71. I bought a brand new MC from advance auto and could never get the brakes to bleed. Had Midas power bleed for $35 (which is unnecessary if everything is in good condition)And 3 blocks later the light was back on and had crappy brakes. Went to Napa and bought a reman, put it on, bled it easily in 10 minutes and have not had an issue since.
Whoever is building the new ones are building junk, get a reman MC.

Jeff