Brake master cylinder

[Jag123]

Hi I’m Paul thanks for the add! I was recently bleeding the rear brakes (middle reservoir) on a left hand drive 1969 4.2. I connect the pressure bleeder and when pressurising, the fluid rose in the other brake reservoir to the point of overflow. I always thought the circuits were completely separate? I can only assume something has failed in the master cylinder perhaps? Any thoughts much appreciated!

- engine was off and all bleed nipples were closed. Thanks!

[mgcjag]

They should be seperate but look at the diagrams in the service manual they both join at the servo.....ditch the pressure bleeder its a good gadget for getting break fluid everywhere.....no reason why old school standard bleeding won't work.....maybe your pressure bleeder pushed fluid past the servo slave seals?.... Steve

[Jag123]

Good point I’m going to leave the pressure bleeder alone from now on! I recon it’s probably the seals on something as before I started work I changed the fluid in the reservoirs as it was dark coloured and full of black rubber particles. I’m thinking to just change the master cylinder and slave cylinder as a precaution to flush out any further faults

[Series1 Stu]

It sounds as though your seals have failed anyway, if they are breaking down and contaminating the brake fluid.

You could just replace the seals if the cylinder bores are ok.

Series 2 dual circuit brakes can be notoriously difficult to bleed, especially when you've rebuilt or replaced both master and slave cylinders.

Regards

[Jag123]

Yep I recon you’re right the seals have definitely had it. As Steve says the bleeder has most likely pushed the fluid past the servo slave seals.

I had a look this afternoon and adjusted the reservoir levels back to normal. Brakes have a normal pedal feel for now so the plan is to change both the brake servo assembly and brake master cylinder over the winter.

I noticed SNG Barratt do both complete units so I’ll most likely go for a straight swap over and have the original parts reconditioned at some point.

Thanks!