Master cylinder or air in brake system?!

[chuggers72]

OK - I've had a trawl through previous posts and found some good info - however not the right info!!

Over the past couple of weekends I have:

Replaced front pads
Replaced seized caliper on o/s front (late Girlings - one piston, one bleed nipple)
Serviced n/s caliper
Fitted new stainless flexi's to front
Flushed/bled system with new fluid in correct order (ie back to the front)

However, I still had a spongy pedal with play all the way to floor. So:

Re bled system - no sign of air
Bled clutch nipple (no air but fluid was almost black so glad I did it!)

Still spongy to the floor.

The pedal firms up well after 4-5 pumps and seems to maintain its pressure until you let off. When van is running, pedal is even spongier with almost no resistance!

I'm now wondering if I have damaged the seals in the master cylinder by pushing the pedal to the floor during testing.

Is there some kind of diagnostic procedure I can do to tell if its either air or master cylinder at fault??

Cheers - all help appreciated

[kevtherev]

So firstly what bought on this brake refurbish?

[chuggers72]

Van was pulling to one side so changed the caliper and put new pads in . Realised a hose was close to falling apart so ended up going stainless. ie one job leading to another as usual

[kevtherev]

Van was pulling to one side so changed the caliper and put new pads in . Realised a hose was close to falling apart so ended up going stainless. ie one job leading to another as usual

and the pedal was fine before this then?

[California Dreamin]

Obviously, fitting new brake pads to older worn brake discs is always going to make your pedal a little spongy but obviously not to the extent you have described. Master cylinder damage is a possibility but if it pumps and holds pressure, unlikely. Under pumped pedal condition, your foot would still sink...this still sounds like air in the system.

[kevtherev]

Obviously, fitting new brake pads to older worn brake discs is always going to make your pedal a little spongy but obviously not to the extent you have described. Master cylinder damage is a possibility but if it pumps and holds pressure, unlikely. Under pumped pedal condition, your foot would still sink...this still sounds like air in the system.

I agree martin.
I like the self bleed type using pressure to force the fluid through, with a depressed pedal.
I would think the air may be in the master cyl

[chuggers72]

Yes - pedal was fine beforehand.

Agree - I was expecting a little more 'sponge' fitting just the pads but like you say not to the degree it currently is!

Ah Kev something you said has me thinking!!! I have bled with under pressure (about 5 psi) but not done it with the pedal depressed! Maybe that's it?........

Was also thinking maybe I should increase the pressure a tad (to maybe 10 psi) - can air bubbles be stubborn to get out sometimes?

[kevtherev]

Ah Kev something you said has me thinking!!! I have bled with under pressure (about 5 psi) but not done it with the pedal depressed! Maybe that's it?........

Was also thinking maybe I should increase the pressure a tad (to maybe 10 psi) - can air bubbles be stubborn to get out sometimes?

I know the method is to pressurise and then open a nipple.
But there's no harm in giving the pedal a little push to be sure there's no air in there.
I've put a litre of fluid through the brakes before now.

One other thing.
pull the hand brake on and then press the brake pedal, tell us what that does

[chuggers72]

OK - tried with and without the handbrake (engine not running).

The difference is marginal however it feels like it becomes firmer quicker with the handbrake on.

[chuggers72]

OK - to continue this thread I'm after some advice.

I've bled the brake system well - flushed through 3 litres of fresh fluid throughout but still a very spongy pedal (as before). I'm thinking (and agreeing with others) that there may be air in the master cylinder.

I've read about bench bleeding the MC but can you do it in situ? If so - how?