got a 93 3/4 ton 4 wheel drive automatic truck here with a new booster and master cylinder, abs dump valve and proporting valve. with recent brakes all the way around loaded calipers new rotors new wheel cylinders lining and drums. the pedal bleeds off and goes to the floor. i adjusted the rear brakes on it also. this thing cant keep a good pedal and has had a history of it. we pressure bled the brakes and got some what of a good pedal and then tryed bleeding it again normally and lost everything. this thing id driving my crazy. no fluid leaks anywhere the only thing i havent done it pull the drums to see if anything has happened to the rear brakes. that may be my next step, anybody had a problem like this befor and solved it. the truck stops fair but the pedal bleeds off nad goes to the floor. any suggestions or ideas would be awsome thanks.

 

also with normal bleeding it has very low flow it any at times.

 

it is possible you have a collapsed brake line. The fluid can soften the inner walls of the brake lines over time, and they can collapse in on themselves.
the symptoms are very similar to having air in the system.

carefully disconnect each line and flush it, tehn try bleeding them again, look fro low volume from teh lines, or low pressure. you may also see the black from the rubber being washed out in the fluid. If so, replace the lines.
of course, you could simply replace them with some good stainless lines any ways

 

Did you MANUALLY adjust the rears? The consensus on the auto adjusters is that they don't work. Most guys adjust rear brakes at every lube oil filter. If the rears are not set just right the pedal will be mush.

JP.

 

no i adjusted them with a brake spoon till you cant turn the tire ( to square the shoes up with the drum) then backed them off till you can feel the brakes bearly dragging.

 

That's what I meant, that's good.

Have you tried blocking off both ports on the master just to double check that the new master is good?

JP.

 

blocking off meaning putting your finger over the ports and seeing if it blows your finger off, i bench blead it and it had good flow. im starting to think its a proporshing valve because its bleeding off like the fluid is bypassing somewhere oh and that is supposed to be a good used proporshing valve cannot get them from the dealer came from a dodge recycler. im researhing what side bore wheel cylinders it is supposed to have in the book it doesnt show a 12x3 rear brake but it has 12x3 rear brake wondering if it has a 94+ rear end and got the wrong size bore wheel cylinders????? that could cause the problem i am chasing

 

the wheel cylinders are the correct bore so that isint the problem i checked to see how far the push rod it sticking out of the booster and it is sticking out the correct length or within thousandts of an inch.

 

No, I made up plugs and caps out of old pieces of brake line so that I could block off lines, MC's, etc. to chase brake problems around.

Fluid has to be moving somewhere if the pedal keeps going down. You can find where by isolating parts of the system in a logical way. Its lots of monkeying around, but it is cheaper than randomly throwing new parts in and you will eventually find the problems.

If you search around you will find more than one story of folks putting in brand new MC's (and other components for that matter) that were bad.

You might also try (temporarily) bypassing the RWAL (dump valve?) thing just to see what happens.

Good Luck and keep us updated,
JP.

 

have you bled the rwabs valve as well. It has a bleeder screw as well.

 

yea i did that

 

capped off the master and the pedal was as hard as a rock then started to bypass the proportoning valve and had to move on to another job so ill be finishing doing that tommrow and let you all know. jp you have been a great help i appericate it im sure you know how it feels when you are chasing your tail and getting frustrated and not getting anywhere and cant think of ways to diagnose the problem (putting the heads back on a dmax in the back of our mind dont help matters much with a customers that wants it now and you dont have the parts to finish it)

 

well i got the proportoning valve bypassed and had a great pedal but a little spungy like had a little air in it and i bled the brakes and had good flow all the way around then went to bleed some more and lost everything has an even worse pedal now then with the proportining valve. any thoughts? bypass the dump valve now?????????

 

Yes, try that. Much more common than the proportioning valve causing any trouble at all.

 

Seems like I remember that the RWAL can fail and depending on which part fails (or mode) can cause spongy brakes. A few simple fittings can bypass the whole thing. Good luck