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MG MGA - Master cylinder queries?

My local parts supplier has said that the inlet and outlet check valve in the head of the brake master cylinder is not needed for disc brake MGAs (they supply two types of overhaul kits - with and without the check valve). I don't see any mention of this in the WSM or on Barney's site. They claim it causes disc brakes to drag! My disc brake car has had the check valve fitted for years with no apparent problems. Any comment?

Also what is the diameter of the compensating orifice hole in the master cylinder - the pin hole at the bottom of the smaller hole of the two in the photo attached - initially I thought this hole was blocked on my MC until I shinned a light down the bore - I can imagine if this hole is blocked or not installed after sleeving of the master cylinder, it could cause the brakes to drag, as has been reported previously on this site (I assume this is the reason some pistons required shortening to ensure this compensating orifice hole is in front of the master cup.

Regards
Mike

Mike Ellsmore

That "inlet and outlet check valve" (another bad name for it, but better than non-return valve) is a necessary part of the braking system. This is the bit that allows you to "pump up" the brakes in case of a low pedal condition. For safety's sale, do not leave it out.

There is a lot of chat about 8-psi residual pressure in the line retained by this valve. This is partly true. When you have high pressure on the system to activate the brakes, then take you foot off the pedal suddenly to release the brakes, this special valve will pop off to let some fluid return instantly while retaining 8-psi in the line. That's not enough pressure to stop your car, bit it does keep brake shoes in contact with the drums momentarily to allow pumping of the pedal.

The detail that is seldom mentioned is that the 8-psi residual pressure is supposed to dissipate after a few seconds to allow full release of the brakes so they don't drag. Some recent issue replacement master cylinders are defective in this valve, permanently retaining the 8-psi without allowing it to bleed off. The fix is quite simple, one tiny little drilled hole in the right place will do it. See details here:
http://mgaguru.com/mgtech/hydraulics/ht116.htm
and here: http://mgaguru.com/mgtech/brakes/bt101a.htm
Barney Gaylord

This thread was discussed on 16/08/2010

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