mot fail rear brakes only. help please!

[jware92]

Hi there, had the vtr in for the mot today. Its failed on both the rear brakes, little or no braking effort. Ive had new discs pads and calipers fitted too. Ive been told it could be the master cylinder. If that was the case wouldnt it affect the fronts? I think it could be the bias valve but its free. Do i need to tighten thd adjuster or loosen it to get more braking to the rear. The last owner may have slackened it off i think.

[adamskiTNR]

Im not sure which comes first int he master cylinder, front or back. If rear is first then the seals may have gone in the first chamber. If the seals are still good for the second chamber, for the fronts, they will still work fine. I wouldn't bet on it though, not a common problem

[saxojh]

I would have said leaky wheel cylinder would cause loss of pressure which would only affect the back. I think the front comes on first as most cars dip at the front underbraking indicating the front brakes are on. As said you need to check the seals but they can be almost outrulled by checking the fluid level as can the wheel cylinder. If it hasnt dropped its probably not a cylinder or seals.

[jware92]

thanks, doesnt seem to be losing fluid and i have new calipers on the back, dont want to go buying a master cylinder if its something else

[Stevo123]

Originally Posted by saxojh

I would have said leaky wheel cylinder would cause loss of pressure which would only affect the back. I think the front comes on first as most cars dip at the front underbraking indicating the front brakes are on. As said you need to check the seals but they can be almost outrulled by checking the fluid level as can the wheel cylinder. If it hasnt dropped its probably not a cylinder or seals.

He should have disc brakes on the back (VTR) so no wheel cylinders.

Quite a problem this one, I would be looking to loosen the pipe from the rear brakes while a mate holds down the brake pedal to see if fluid is reaching the rears under pressure. If no pressure then look to change over master cylinder but as mentioned already this is quite unlikely.

Double check bias valve opertation, again this could have an internal fault not evident from just looking at it.

I have heard before of flexible hoses going bad and blocking fluid pressure so check carefully the rear flexies.

[jware92]

ok. when the brakes were bled there was plenty of fluid coming out, can see what the problem is. not really sure how to do the bias valve?

[dt_]

I once failed on this and the guy who used to work on my car said he 'wound the pistons in and out' a bit and it passed, not sure what he meant by this, havent touched the rear brakes for years and never had a problem

[Gareth_R]

pistons a bit seized, winding them in and out a bit will loosen them off so they move more freely

[mervster]

My VTR had the same problem and it was the adjuster on the Bias Valve, wasnt set right by the garage that lowered it... I got a new Adjuster from Citroen (£7 as mine was seized solid) wound it in a bit further than before so it was shorter and the brakes were perfect..