Spongey brakes! Not again!

[RossyB108]

Do you think I could have damaged the new master cylinder when I fitted it? How fragile are they?! I bought the new one becuase I thought the problems were because of the cylinder but I have the same issue even after fitting the new one! Just seems unlikely to have developed the same fault, it'll be expensive and VERY annoying if I change the MC again to find it's doing the same thing.

I'll jack up the rear and bleed, but it wont solve the problem as it'll be the same again after a couple of weeks.

I was thinking of blanking off the rear brakes from the front union/brake pipe splitter near the engine bay somehow, to isolate the rear. That way it should narrow down the problem.  If the front of the car is fine, at least I'll know to buy a new bias valve and or new/upgrade calipers.

Dam it.

[clipperjay]

At this stage If the M/C is new send it back to supplier to check it out cost of postage if its faulty then they will exchange it for you hopefully?
Otherwise I don't see you having any pressure equipment to check for leaks?
The SERVO is very tough, the skin on the cylinder would need a drill to puncture the surface for it to loose pressure!

Is the servo NEW? Did you use brand new rubber seals to mate the M/C to servo?
 

[RossyB108]

No, the servo looks original.

MC came from GSF and it was the better quality option (22mm). I used a new seal that came with the MC but the surface of the servo wasn't great. Although saying that, after a bleed the brakes work perfectly for at least a couple of weeks and I'm guessing the MC to servo seal is seperate from the fluid? So air is getting into the fluid somehow.

Thanks

[clipperjay]

What about the resivior seals that sit on top of the M/C?

[RossyB108]

A new pair did come with the new MC and it also came with some lubricant to put on them before fitting the res.

It's a bit tight rought there but I can't see any obvious leaks.

[clipperjay]

I know this sounds simple you have taken the resi cap off to check the level and not just from the outside?
All I'm saying is it might not be clear visually if dirt is giving you impression of the level!
Before parting with any money I would make sure every caliper piston winds back ok and all rubber seals are in good order.

It could still be M/C or Servo issues here?

[RossyB108]

Ok thanks clipperjay, I'll check the calipers etc. and the levels...

[RossyB108]

Update.

Two weekends ago I replaced my rear calipers with mk4 ones and fitted new braided hoses all round, (except for the small hoses that fit over the rear beam coz I couldn't get in there and the fittings look well corrdoded so left those ones for now).

After fitting and bleeding, the peddle felt firm and sharper, But over the days since, I have noticed the very gradual spongyness creeping back again, I'm guessing within the next 2 or 3 weeks depending on the amount of driving I do, the brakes will have gone again.

This is seriously getting on my wick now.

I've replaced:
Master cylinder
Rear brake pipes
Hoses for braided ones (except rear beam hoses).
Rear calipers for mk4 ones

 - There are no leaks anywhere that I can see
 - Air is getting into the system and takes about 4/5 weeks before the peddle is near the floor
 - A double pump on the brake peddle makes the brakes work and feel firm and normal (but I shouldn't have to double pump every time I need to brake!)
 - The brake fluid level doesn't change (that I can see)
 - The rear brake compensator valve works and isn't seized or leaking

My next plan is to have the rear braided hoses fitted and the corroded union and pipes replaced at the back near the rear calipers in case that's where air is getting in.

After that if the problem is still there, I guess the only parts left to replace are:

 - Rear compensator valve
 - Front calipers
 - Servo
 
What a joke, am I really the only person this has happened to???

[RossyB108]

Just giong back through some old posts to update.

I found the problem in the end - I was just being a dumbass.

I opened up the rear bias/brake compensation valve with some mole grips and pressed the brake pedal hard for a few seconds.  Found brake fluid coming out of one of the rear calipers hose connections. I did it up nice and tight, re-bled and bingo no problems since (5 months).

The bios valve only lets though a little fluid (depending on how it's set up) so I wasn't getting any leaks but air was slowly getting in the system from the rear caliper over the course of a couple of weeks.

At least I have a mint brake system now!

[clipperjay]

  everyone gets caught out over small things funny I'm having brake issues aswell!
I'll sort it out but very slowly indeedie!
Good job though for your patience on it