in the past year i have replaced shoes, pads, calipers, booster....i adjusted rears...bled system

ever since i replaced the booster though i can't get consistent brake pedal feel...I will apply the brakes...do it three times in a row I can feel that the brakes don't let go...its avery slight feeling and feels like i have some resistance and brakes are slightly still applied...

If then I pull up on the brake pedal (even though there is like 5 mm free play) a couple of times the truck suddenly relaxes and it feels like weight has been thrown of its back...

here is the odd thing

i have bled the system numerous times including takign off the master cylinder recently, bench bleeding it and then bleeding the system....and each time i bleed it, the brakes work great with a springy return and very responsive....and after 15 min of driving and 20 or so stops it goes back to what i described

btw....no fluid leaks at all

what to do??....is it the rod on my booster?

 

Sounds like a bad master cylinder to me.

 

yeah thats what I'm thinking as well....I did take it apart though before the bench bleed and all seals looked great..springs were great and no scratches inside....BUT one thing i noticed is that the outlet for the 2ndry piston (furthest one) was leaking on its own after i primed the system...isnt it supposed to have a checkvalve?

 

I don't know about the check-valve??? I have never torn into one before.

But those are classic sign's of a bad master cylinder.


















Plus you've changed out everything else in the brake system.

Mark

 

grrrrr I know

 

anyone ever experience this??....oh yeah at the same time i replaced the booster I also upgraded to the GM one ton wheel cylinders for the rears...so maybe that has something to do with it too?

pedal does not sink to the floor at a light and I do not have to pump it to stop or anyhtign like that.....after a few stops the brakes just dont seem to let go complely after i let go of the pedal...if i pull up on the pedal (even though it is fully retracted) I can feel that it loosens up...

 

Have you replaced the rubber flex lines? They can come apart inside and block flow , usually they result in a pull in the brakes, but if they blocked flow in the other direction ,they could hold some pressure on the brakes. The bad part, is it will be cheaper for you to throw a rebuilt master cylinder on it , than to replace the 3 (or 4 with 4 wheel abs) rubber lines.

 

have you been playing with the push rod?

 

If it was the first time for you bleeding the brakes after swapping cylinders and you used the brake pedal to bleed and pushed the pedal all the way to the floor you may have likely damaged the master. I'm not sure you could see the damage just by looking. The price of rebuilt masters has come down so far nowadays that I no longer go though the hassle of rebuilding them myself.

 

Good point infidel, I was always told/taught not to bottom out a master cylinder when bleeding because damage can occur.

 

yeah I'm pretty sure my father bottomed out the pedal repeatedly furing the bleeding process...agh....interesting question on the side...why is it that they recommned pushing the piston fully in with the the phillips screwdriver during the bench bleed?

 

you cant push that screw driver as hard as the pedal, so you wouldn't be putting the leverage on the master like the pedal does. Just my guess?

 

thanks guys...I'm gonna double check the lines for kinks and get a new master on monday...they're like 47$ at napa (rebuilt)

 

Two posibilitys.The hydro boost is keeping the master cylinder partially engauged.Was there an adjustable rod on the master cylinder side of the booster?If so it may need the amount it sticks out adjusted.Or the hydro boost may just be starting to apply itself on its own.

 

ok...i dunno how this happens but it was the rod on the booster..i guess the way it was rebuilt either they used a rod that was oto long or had the wrong clearance and it sticks out a bit more than necessary....the thing was creatign a preload on the master cylinder....so i added a couple washers where the master bolts to the booster until i had it sit noice and flush and boom....that was it

another one for the books....