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MG Midget and Sprite Technical - Brake Trap Valve - necessary or not?


I'm fitting a non servo dual master cylinder in an old MGB (similar to the type used on later 1500 midgets. But, the Trap Valves (that sit behind a reducer in the output ports on a spring) are rather hard to come by.

I just wondered if anyone had any experience of running cylinders without these. Specifically, is it causing air ingress from the wheel cylinders (i.e spongey brakes after a while that are cured by a re-bleed)?

Any thoughts greatly appreciated.
M Blacher

used to use a dual master on the racer but it *always* was spongy no matter what we did; another racer said the same so we binned it and went for separate Wilwood m/cyls and a balance bar.
David Smith

Those valves are there to hold a bit of pressure on the lines, specifically to keep the rear drum springs from collapsing the shoes all the way and causing a longer than needed pedal movement. If you keep the rear drums well adjusted you shouldn't have any problems with pedal movement, but if you should you can always install an aftermarket in line residual pressure check valve if you want. Wilwood makes these in a 10psi size for drum brakes and a 2psi size for discs.
B Young

Not quite.The shoes should (and do) always retract to rest position. The RP valves are to maintain some pressure to keep the WC cups expanded and not leaking, either fluid out or air in. The RP valves also help when bleeding, as they keep the fluid/air from just moving to and fro when you pump the brakes. This has obvious implications for pumping up the brakes in the event of partial failure.

FR Millmore

This thread was discussed on 30/11/2011

MG Midget and Sprite Technical index

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