405 Mi 16 Master Cylinder

[fiji bob 0]

I think mines leaking internally, the brake pedal feels firm with the engine off and sinks slightly when you start the engine but sinks almost to the floor if you use moderate pressure with the engine running also it seems to hold pressure if you repeatedly press the pedal, i was wondering if there are any other cars that use a bendix 22.5mm master cylinder? the car has the seperate bendix abs unit. Is it common for the rear brake compensator spring to break on these cars? how much eddect does it have on the rear brakes under normal conditions ie not loaded down?

[welshpug 1,645]

quite common to have issues with the rear compensator, it has quite an effect on the braking of the car, as for the MC I'd personally just buy a new one as it is such a vital part of the braking system.

 

the compensator can only be sourced complete and is about £70 from GSF for a branded unit, MC is about the £30 mark last time I bought one.

 

transformed the braking system fitting those two items alone.

 

do you have the Bosch additional ABS then?

[fiji bob 0]

quite common to have issues with the rear compensator, it has quite an effect on the braking of the car, as for the MC I'd personally just buy a new one as it is such a vital part of the braking system.

 

the compensator can only be sourced complete and is about £70 from GSF for a branded unit, MC is about the £30 mark last time I bought one.

 

transformed the braking system fitting those two items alone.

 

do you have the Bosch additional ABS then?

 

no its the bendix additional abs

[fiji bob 0]

Replaced the master cylinder bled it making sure the rear valve was opened but it's still the same the pedal can be pumped up but sinks right down if you hold pressure on it can a faulty servo cause these symptoms?

[taylorspug 7]

Funny i had the same thing with the same master cylinder, and a new master cylinder fixed it. I doubt its the servo, but to check it just try the test with the engine off and the vacuum pumped out of the servo. Obviously you will have to press alot harder on the pedal though!

[fiji bob 0]

We swapped the master cylinder for a brand new one from euro car parts it's a 4 port type rather than the original 2 but seems alright, the pedal pumps up pretty solid with the engine off but sinks when you start the engine it's stayed the same with the new master cylinder, something I noticed if you use a pressure bleeder and leave it connected with the engine on the pedal stays firm. I'm pretty sure it's bled properly the fluid was squirting nicely out the back calipers.

[fiji bob 0]

Alright we clamped both flexible rear hoses and rechecked the pedal it's now fine with the engine running, it's something on the nearside back that seems to be causing the dodgy pedal, that caliper was replaced a while ago and all metal lines at the back have been replaced. Can a caliper cause the symptoms or the valve at the back?

[welshpug 1,645]

could be either, though if its limited to one side its more likely to be the caliper itself, is the opposing front caliper fine? (i.e same part of the M.C)

 

they can be real buggers to bleed at the back.

Edited September 21, 2009 by welshpug

[Miles 331 1 Cars]

I;ve known of the callipers to be taken off and held upside down due to the bleed nipple location

[welshpug 1,645]

yeah, haynes manual says to do it that way, block of wood in the caliper to stop the piston from coming out, also suspension compressed to keep compensator valve open.

[fiji bob 0]

Might be worth trying the bleed nipple on the newer caliper is in a different location compared to the original

[welshpug 1,645]

sounds like a vw golf caliper then, they are very slightly different.

[fiji bob 0]

Is it possible that one side of the valve might be restricting more than the other causing the excess pedal travel while that side catches up? I'm not sure how the master cylinder actually works just a thought

[fiji bob 0]

Turning it upside down worked, back to normal, thanks chaps

[welshpug 1,645]

yay!