Modification of Brake Master Cylinder for use with Disc Brakes

R H Johnston

The master cylinder has a valve at the far end which helps to make bleeding the brakes easy by preventing fluid coming back when the brake pedal is released. It also retains a small residual pressure which opposes the return springs on drum brakes and helps to keep the shoes in the optimum position, even of they have not been adjusted for a while.

This residual pressure can cause problems with disc brakes, because there are no springs on disc brakes to draw them off against this pressure. As a result the brakes may stay on slightly all the time. This not only impairs performance and results in rapid wear of the pads (I got through a set in less than 5000 miles), but it also leads to the pads becoming glazed which reduces the coefficient of friction so that a very much higher pedal pressure is required to stop.

Hence when disc brakes are fitted in conjunction with the standard A30/35 master cylinder the master cylinder should be modified. The master cylinder should be dismantled and the small rubber valve cup in the metal valve body at the far end of the master cylinder should be removed, or, better still, a small hole made in the rubber valve cup in line with one of the holes in the metal valve body. This hole should be as small as possible consistent with being a visible hole. The master cylinder should then be reassembled as normal.

If the rubber valve cup has been removed completely the brakes will be difficult or impossible to bleed by pumping the brake pedal, and the modern type of bleeding equipment which uses the pressure from a car tyre should be used instead. If the hole has been made in the rubber valve cup instead, the pedal pumping method will work provided the hole is small and the pedal is allowed to return as rapidly as possible when the pedal is released.

Whilst great care is taken to ensure that the technical information and advice offered is of the highest standard, neither the author, nor The Austin A30/A35 Owners Club, or the Officers of the Austin A30/A35 Club accept any liability at law for any death, injury or loss whatsoever arising from the use of advice contained in this article.