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MG MGB Technical - Centering Pressure Fail Shuttle

I replaced the "O" rings in the brake pressure failure switch shuttle, and bled the brakes. I stopped the leak and have a good pedal, but the pressure failure light remains on (unless I unplug it). I suspect the shuttle is off to one side or the other, triggering the switch. I found one service manual that said, to open a bleed nipple on the 'opposite' half of the system, and slowly depress the pedal until the light goes out. I am not sure what the other half really means, has anyone out there tried removing the pressure fail switch and center the shuttle with a tiny screwdriver? This would seem easier than rebleeding the system, especially since I am happy with the feel of these brakes.
Joe

I had read that while bleeding the brakes you should loosen the fitting the switch plugs into and that will center the shuttle while you bleed.
william fox

The failure switch is supposed to detect a marked difference of hydraulic pressure between the front
and rear brake systems. The "opposite half" (as written in your manual) means opening a bleed screw
that is opposite to the side where the piston had shifted to.

Remove the brake failure switch and peer into the opening (with a flashlight). You should be able to
see the "waist" of the piston centered inside the union.

If the shuttle piston is shifted towards the rear brake lines - then open a bleed screw at either one
of the front calipers and press on the brake pedal. This will pressurize the rear brake system (only)
and will move the shuttle piston towards the front brake lines.

If the shuttle piston is shifted towards the front brake lines - then open a bleed screw on either one of
the rear brake cylinders and gently press the brake pedal. This will pressurize the front brake system
(only) and will move the piston towards the rear brake lines.

Remember, you only want to do this just enough to center the shuttle piston inside the union.

Having said all that...I've used a small screwdriver to center the shuttle piston while I had the assembly
mounted on a bench vise. I see no problem in doing this while it is installed on the car.
Daniel Wong

You didn't mention the year, but ditto to Daniel's suggestion to simply use a screwdriver to recenter the shuttle.
The fail indicator system on the RB car was certainly not among England's finest contributions to safety. Even though the majority stay leak free for many years, they don't always shift in the event of differential pressure, the warning light assy is too fragile and there is no simple way to test the system short of inducing a fail.
If the o-rings in the RB mc begin leaking thru the switch, you'll eventually lose brakes if not caught in time. Speaking from experience, a freshly reman'd master cyl from a well known supplier puked the contents of its reservoir thru faulty seals on both ends of the shuttle, which exited thru the weep hole in the switch, sprayed in a fine mist onto a freshly painted inner fender in about 15 minutes of stop and go driving. This happened after a series of semi-panic stops to make sure all was well and nothing was leaking. Pedal stayed hard until that first gasp of air hit the lines. Never got a light, although the system had been force fail tested earlier. Didn;t need no stinking light to announce something was wrong- the pedal (and no doubt the wet spot in his shorts) let the driver know that something was wrong just a few miliseconds before the light would have gone had there been a pressure differential... pedal hardened up at the floor, the car stopped before hitting anything. The brake rebuilder was very accomodating and apologetic, and paid for R&R labor and for a respray on the inner fender, even tho the expressed warranty excluded that sort of damage.
Mark Childers

I couldn't center it by screwdriver, so I opened one of the rear bleeders and pressed on the brake pedal by hand until the light went out, closed the bleeder and it seems OK, will test drive to be sure
Joe

This thread was discussed on 21/09/2002

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