Rear Brake Line Pressure

[fliprio 9]

Morning all,

 

I’m having some issues with what I think is low rear brake line pressure. I have a Gti-6 283mm front set-up, with a 406 24mm master cylinder, single line to the rear with a tilton bias/proportioning valve inline, y-piece to the two rear callipers. Compensators have been removed. Before fitting the bigger master cylinder, I was having a lot of rear lock-up problems on sprints and the pedal having too much travel.

 

It was Mot time again, and the rear brakes would only muster 30kg on the rollers, the handbrake coming in at about 80kg. The bias valve was all the way in.

One calliper is quite new, stripping the rear brakes showed it was fine, no air, there is fluid movement etc. However, with your foot hard on the pedal you can still turn the wheel by hand. The bias valve seems to be working, you can back off the pressure and the wheel is easier to turn.

 

Could this be a master cylinder problem? The pedal remains firm which has me scratching my head somewhat.

 

I don’t remember there being an issue with poor rear brakes at the last MOT, but overall the 24mm master cylinder made the pedal much nicer in feel, but does make the brakes feel a little weaker, which I assume is down to the reduced line pressure of 24 vs the 21mm of the original 205 master cylinder.

 

Cheers, Dan

[welshpug 1,656]

probably air in the lines especially if you changed the master and the handbrake is good.

[fliprio 9]

probably air in the lines especially if you changed the master and the handbrake is good.

 

maybe i need to bleed more fluid through, not a single bubble at the caliper when cracking open the bleed nipples, the master cylinder was 3 years ago, so has been working fine with it on there

[fliprio 9]

probably air in the lines especially if you changed the master and the handbrake is good.

 

also, wouldn't you think the pedal would be spongy if there was that much air in there?

[Biggles 65]

Cable handbrake I presume ?

[welshpug 1,656]

with a front/rear split and no leaks the pedal can still feel quite firm for half its travel.

[fliprio 9]

Cable handbrake I presume ?

yep, standard cable set-up

[fliprio 9]

with a front/rear split and no leaks the pedal can still feel quite firm for half its travel.

 

ok, will bleed a bit more thought and check for no air. I had wondered if the bias valve was knackered, but the guy at tilton said it would only reduce by 60% even if it was knackered.

[Biggles 65]

But surely you'd notice the travel (I always did). Agree double check it's bled up right though.

 

Going from a 21mm to 26mm m/cyl would reduce line pressure by 35% to start with - another 60% on top of that leaves 39% of what you started with ...

[welshpug 1,656]

26mm ???

[Biggles 65]

Oops