Rear brake pressure not releasing.

So still struggling to get this sorted.  

New 9inch servo
New master cylinder from a mk2
Removed the pressure regulators located by the bulk head
Mk2 rear discs
Mk4 rear callipers
Plumbed N/S rear to O/S front and vice versa

Test drive just and slammed the brakes, all pretty straight and rears locking up before the fronts.

Hand brake ok but loose if anything before the test drive.

A 1 mile trip and the rears are binding. Pressure being held on hydraulically. If I release the connection on the m/c then they free up.

The only thing in line on the rears is the brake bias valve. I can remove this and fit some bypass sections if needed but not sure that would be the issues.

Anyone else had these issues doing rear disc conversions?  I am getting to the point of thing it is something more serious in the set up.

Thoughts and experiences welcome.


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Forget their technical name but the rear one way valve thingys need removing when fitting rear discs, if they havent been.  Hexagonal looking connectors down under the master cylinder on the rear brake lines. Believe their job is to maintain pressure in the rear brakes…

abfmk1 said

Forget their technical name but the rear one way valve thingys need removing when fitting rear discs, if they havent been.  Hexagonal looking connectors down under the master cylinder on the rear brake lines. Believe their job is to maintain pressure in the rear brakes…

No, they restrict the flow of fluid to the rears preventing a lockup.  

If you used used hoses that may be an issue as hoses can deteriorate internally and delaminate.  The de-lamination can cause the fluid from returning to the res as it  acts like a one way valve.

Brand new hoses etc.
bled about 5 time so fluid is absolutely fresh all through no debris etc.
brand new callipers not second hand

I going to have to bypass the load regulator by the rear axle and see if that works.

Pressure regulators below the master cylinder have also been removed.

Thx


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You may need to source an adjustable pressure regulator to the rears, as they are a smaller diameter piston, and as you have remove the restriction to the rears at the front they are getting the same amount of fluid to them as the fronts, as the fronts are larger pistons they move a shorter distance and the smaller rears move further for the same amount of push of the pedal.

Makes sense. I shall get them releasing first and then get some adjustable regulators to balance.

Will get a pair as I want to keep them diagonally plumbed.


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Backinblack said

Makes sense. I shall get them releasing first and then get some adjustable regulators to balance.

Will get a pair as I want to keep them diagonally plumbed.


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They are diagonally plumbed to ensure that if you have a leak in one side or the other, you don't loose all braking, and that it will stop more straighter.