GQ brake issues

[Breaker Brother]

Went to put new pads in the front the other day, go to wind in the callipers, left hand one starts leaking fluid everywhere,
no biggy, have a spare so whack it on.
Bled it up, and the Pedal starts sinking

Think I've stuffed the cylinder, so get a new PBR one whack it on, same bloody thing.

Bleed it six ways from Sunday, including the LSPV, definatly no air in the system, nor leaks. still got a sinking pedal.
Double checked all he callipers, made sure all the slides were free, lines & hoses in good nick, can't find a thing wrong, still got a sinking bloody pedal.

Now I'm starting to think the reaction diaphragm in the booster may be shot, it holds vacuum but that doesn't really mean shit.

What you guys think ? I've bout had it

[chunks]

So with the engine off can you get a firm pedal or does it sink?

[BadMav]

You may have some crud in the system and when you pushed the piston in it's pushed the crud back into the master cylinder and under the skirt of the seal on the piston. You might get away with a quick pull down and clean out.

[Breaker Brother]

So with the engine off can you get a firm pedal or does it sink?

Yup, no vacuum to the booster and it's rock solid
Interestingly, the vacuum pump generates a bit over 30" of vacuum, I woulda thought that was a lot

You may have some crud in the system and when you pushed the piston in it's pushed the crud back into the master cylinder and under the skirt of the seal on the piston. You might get away with a quick pull down and clean out.

The whole thing has been Bled multiple times, both before and after fitting the new (2nd hand) caliper

[chunks]

If you have a nice firm pedal with the engine off then you shouldn't have any air in the system. Does the pedal go all the way to the floor or just sink a bit? I always find the pedal is a little bit soft when you first put new pads in, have you tried going for a drive and bedding them in?

[Breaker Brother]

after driving, the first hit of the pedal is spongy as you've described. but come to a complete stop and the pedal will slowly continue sinking, close enough to the floor that it may as well be on it

[OLRacing]

after driving, the first hit of the pedal is spongy as you've described. but come to a complete stop and the pedal will slowly continue sinking, close enough to the floor that it may as well be on it

Are you losing any brake fluid from the resevoir?

[Breaker Brother]

after driving, the first hit of the pedal is spongy as you've described. but come to a complete stop and the pedal will slowly continue sinking, close enough to the floor that it may as well be on it

Are you losing any brake fluid from the resevoir?

Nope

[V.W.Dave]

You may have some crud in the system and when you pushed the piston in it's pushed the crud back into the master cylinder and under the skirt of the seal on the piston. You might get away with a quick pull down and clean out.

He replaced the master after pushing the pads back so what you said could have never taken place.


You replaced the master with a USED or REBUILT one? If it is just a used one it may be shagged?
Have you had a close look at all the brake lines to see if any of them are swollen?

[Breaker Brother]

Fitted a Brand new PBR unit

[purplebus]

had exactly the same problem and chased it for ages... .
rock hard pedal when motor off and once started pedal would go to the floor, happened when i changed booster. tried about 3 or 4 diff boosters etc, etc etc.

a mate followed the brake lines towards the back of the car and about half way along there was some type of stupid cylinder no one had seen before with a bleed nipple on it. bled this nipple and it fixed it.we had never seen one on a gq and havent since.seen 1 on a mq super deluxe i had but no gqs other than mine. rolled the truck a few months later so cant post a pic but was about 3" long and 1" round from memory.hope this helps.

[V.W.Dave]

Or you could take it to a mechanic and get them to look at it for you

[Breaker Brother]

had exactly the same problem and chased it for ages... .
rock hard pedal when motor off and once started pedal would go to the floor, happened when i changed booster. tried about 3 or 4 diff boosters etc, etc etc.

a mate followed the brake lines towards the back of the car and about half way along there was some type of stupid cylinder no one had seen before with a bleed nipple on it. bled this nipple and it fixed it.we had never seen one on a gq and havent since.seen 1 on a mq super deluxe i had but no gqs other than mine. rolled the truck a few months later so cant post a pic but was about 3" long and 1" round from memory.hope this helps.

That's the LSPV, I've bled that too............

[AUGTREE]

I had the same issues. Ended up making my own suction bleeder using a small glass jar with steel lid, inlet and outlet ports in lid. And used suction end of compressor to suck the fluid through the glass jar. Rock solid brakes now. It took me two days to get to that point. I was about ready to put a match to it!!

[chunks]

Did you bleed the master cylinder before you installed it?

[love ke70]

like chunks said, bench bleed master, the do it again once its in...

[mkpatrol]

like chunks said, bench bleed master, the do it again once its in...

X10000000000, bench bleeding is imperitive before fitment to the car. You can bleed til the cows come home while its intalled & still get no pedal.

[christo400]

Just a thought make sure the bleed nipple is at the top of the donner caliper - ie wrong side caliper fitted causing air to get trapped in caliper if that is ok then try pushing both caliper pistons rite back to reseat the seals - good luck