my brake bleeding problem, on a gmt400 tho.

[Swims350]

I know technically this should be in th eother vehicles, but I figured it's for brakes maybe get a better/faster answer in here.

As you know my brakes are giving me fits now, and I removed the ABS, on this truck is RWAL only, with a isolation/dump valve, that's removed, so it's like old school squarebody power brakes.

This is my problem...


I bleed either side, starting on the PS rear of course but either one does this same thing..

bleed them once, works good, twice, still good, 3rd time, 4th. time any after barely get anything coming out and the pedal inside the truck moves less and less. The first time we'll say it moves halfway down, that's it, maybe more, 2nd time it'll only move 1/4 or less, 3rd. 1/8 or none and etc.

ok so I try to bleed them up front, at the prop. valve or at the MC. So this is what happens, pump the pedal and hold, crack the front line loose at either the MC or the Prop valve, fluid comes out, pedal travels all the way to the floor as it should.

crack the rear line loose at either the MC or the prop valve and the pedal moves only an inch or less, and does the same crap, 1st try an inch, 2nd try 1/2 3rd try nothing.

now bleeding the front brakes at the calipers the pedal will go all the way to the floor as it should.

one last thing, when the MC cover is off and you hit the pedal gently and watch it, the rear most part(for the front brakes) shoots up a stream of fluid as it should, the front tank(for the rear brakes) does nothing, no stream comes up or anything.


I think the master cylinder (MC) is shot, what do you think?

 

[crazy4offroad]

Sounds like the MC to me, if you leave the lines connected and unbolt it from the booster see if you have any brake fluid between the booster and MC, usually a dead giveaway of MC problems. If it's never been changed it would be a good idea to change it anyway.

 

[Swims350]

trying to gravity feed this piece a crap, had it going good, almost sucked it dry tho, then it wouldn't start again, it didn't go dry but close.

Now I got it going again, tons of bubbles in the 1 foot clear tubing I got going into a catch jug, then it just stops, no bubbles, no fluid, mc is full.

master cylinder lid is off and the PS rear is the only one open.

 

[crazy4offroad]

Sometimes they can suck air from around the bleeder screw when you gravity bleed so dont always believe the air is coming from the system. Try pushing the bleeder screw down (while loose) and see if the bubbles stop or diminish, while fluid still runs out.

 

[Swims350]

might have to try it. it just stopped bleeding tho, wouldn't pull any fluid thru at all.

if I pump the pedal it gets hard, then I left off and hit it and it just drops to the floor. When I pump them it goes down about halfway or so and is hard.

I removed the MC from the booster with lines attached and they was no leaks at all.

 

[Swims350]

alright I uncorked the lines from the MC, used the fitting/nipples that come with them for you to bleed them, ran the hoses into the tank, watched the bubbles and such.

They did bubble, but what was weird is the front tank(rear brakes) at first did nothing only dripped fluid out, then after awhile started to pump fluid good, but only for the first half of the pedal travel. once it got to half way or so it stopped doing anything and the rear tank(front brakes) it kept pumping fluid from beginning to end, and the rear tank itself kept pushing fluid up thru the bottom of the tank where as the front did not. It never pumped anything thru the bottom up thru the tank like the back did.

I'm pretty sure they should both pump fluid up like that front and rear at all times.

my guess is that the front seals are messed up aloowing fluid to go to the back and those seals are fine. Whcih means front is great back has nothing.

That's just my guess. I'm going to havew my old man check it out tomorrow, see what he thinks and looking for any info or help you guys might have.

 

[Swims350]

got them to gravity bleed finally, did all four bleed a quart of fluid thru each, still no change.

the PS wheel cylinder kept having bubbes in the line, no change grabbing it squeezing it or moving the bleeder around.

DS did not, had solid strem the whole time.

PS caliper had air bubbles too, alot, same deal moving made no difference.

DS had some, every once in awhile but mostly a solid stream.

Still no change, guess I'm going to have to change the MC and see what happens.

 

[89Suburban]

Did you look at this?

http://www.classicperform.com/TechBook/BrakeTroubleshoot.htm#testprop

 

[Swims350]

no but I am now.

I do plan to try and block the MC and check it for pressure.

 

[Swims350]

well it seems the MC is NOT junk, plugged both ports, was hard like it should be, plugged the rear line port, hard as it should be, plugged the front lines only allowing the back to work, and it was soft, both lines hooked up and working it's soft. I could hear my prop valve reset as well.

So now I have to make sure and bleed it good and get it out.

 

[89Suburban]

well it seems the MC is NOT junk, plugged both ports, was hard like it should be, plugged the rear line port, hard as it should be, plugged the front lines only allowing the back to work, and it was soft, both lines hooked up and working it's soft. I could hear my prop valve reset as well.

So now I have to make sure and bleed it good and get it out.

Good luck dude.

 

[Swims350]

I still can't get this piece a crap going. we bled and bled, got good fluid no air, nothing, half pedal engine off, engine on and it drops to the floor and sets the brake light.

front port for the rear brakes plugged on the MC has full hard pedal off, engine on has over half pedal solid. This thing is really ticking me off. I decided to pop off the lines and blow thru them, twisted the end off the DS line so now I gotta get new lines across the rear end. Who knows what all else.

I also can NOT figure out which way to holsd or set the prop valve to keep it open to bleed it.

 

[jt1200r]

hows your rubber line for the rear brakes. could be deteriated on the inside

 

[Swims350]

fine now, I ended up fighting this piece of crap all day and still nothing.

I ended up trying to take the rear lines loose and blow thru them, twisted the wheel cylinder end off the DS, went to autojunkzone and bought 2 lines one fit the other didn't, what crap. So I have to exchange that line tomorrow and then try and rebleed them once I get home.

 

[89Suburban]

Chris, did you happen to check the condition of the rear steel brake line running past the saddle fuel tank?