Master Brake Cylinder Rebuild - Rear Circuit not Priming

[Hughesy]

Any ideas on this would be much appreciated.

Just rebuilt the brake master cylinder - new rubber seals and a light hone. The front circuit bleed as normal using manual bottle and tube method. However the rear circuit is not priming in the master cylinder. I know this as I have bleed both circuits exiting the master cylinder. The front squirts plenty of fluid but the rear barely produces a drop. I noticed the front piston cups (rear circuit) were fairly tight in the cylinder when I reassembled the unit. Brake pedal will not press all the way to the floor when the rears are bleed. On the fronts the pedal went all the way down as normal. Ideas? Worth trying a pressure bleed??

[72BLUE]

Is you PDWA valve stuck?

Keith

[Hughesy]

No since when I undo the rear circuit directly at the side of the master cylinder I have no fluid expelling when pedal pressure is applied. This means the problem is within the master cylinder.

[72BLUE]

Ah, whoops! Just re-read your initial post, properly this time!

Keith

[therat04]

Any ideas on this would be much appreciated.

Just rebuilt the brake master cylinder - new rubber seals and a light hone. The front circuit bleed as normal using manual bottle and tube method. However the rear circuit is not priming in the master cylinder. I know this as I have bleed both circuits exiting the master cylinder. The front squirts plenty of fluid but the rear barely produces a drop. I noticed the front piston cups (rear circuit) were fairly tight in the cylinder when I reassembled the unit. Brake pedal will not press all the way to the floor when the rears are bleed. On the fronts the pedal went all the way down as normal. Ideas? Worth trying a pressure bleed??

Hello Hughsey did you prime the master cylinder before you fitted it? i.e. in a bucket because its messy, fill master cylinder reservoir and manually pump the piston vigorously until fluid comes out of both outlets, then bung up the holes while you carry it to the car remove bungs and fit pipes, that should do the trick...Steve

[Hughesy]

Hello Hughsey did you prime the master cylinder before you fitted it? i.e. in a bucket because its messy, fill master cylinder reservoir and manually pump the piston vigorously until fluid comes out of both outlets, then bung up the holes while you carry it to the car remove bungs and fit pipes, that should do the trick...Steve

Yes that has helped but only after blocking the exit pipe hole on the backstroke ! Got to fit it back in the car now and try again. Thanks!

[Hughesy]

I ended up buying a bench bleeding kit and bleed the master cylinder using this kit with it on the car. The kit connects the two master cylinder outlets via clear tubes and returns the fluid back to the reservoir. To bleed the rear circuit I kept the bench bleed system connected to the front circuit and then bleed the rears at the wheel cylinders. This gave me full stroke on the pedal. Then connected up the front circuit and bleed the fronts. Pedal is now rock hard! I last did this master cylinder about 6 years ago and never had this problem. Anyway job is done now.