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MG MGB Technical - Hard brakes update

Well after trying everything else I took the brake failure switch out of the failure valve. The brakes work much better and I can stop it much faster than before. The switch had a worn spot (flattened on one side)on the part that goes into the valve. Any ideas on how that could happen or why it would affect the brakes. I will order a new switch if I can and see what happens. Thanks for all the advise from everyone.
Fred
FRED DOHERTY

Fred: See my post on your earlier thread. If the valve is screwed right up against the pedal then you have eliminated all pedal free play. The result is the same as riding the brakes all the time. The system overheats and the fluid can not expand. Result: rock hard pedal, little braking.
The manual gives the procedure for adjustment and free play. When I install the switch I will turn it in slowly until the brakes just light, then back it off until the correct free play is obtained, then tighten the lock nut.
Regards, Andy
Andrew Blackley

Fred: I don't have any idea why removing the switch from the valve should improve the brakes
unless there was some trash partially blocking one brake circuit and removing the swith
allowed the shuttle valve to shift dislodging the the trash. I just made some checks on my
spare pressure failure switch and moving the shuttle valve to either limit will not block
either brake circuit. As I understand the pressure failure switch, it's sole purpose is to turn on
the brake failure light in case one of the brake circuits has low or no pressure which in turn
will move the shuttle valve and turn the switch, the switch turns on the brake failure light to
give a visual indication of a brake problem.

However, I did note that the factory shop manual reprint and the Bently Complete Official
MGB do not tell how to centralise the failure switch. That information can be found in the
Haynes MGB repair manual. It's on page 158 in my Haynes.

FWIW, Clifton
Clifton Gordon

Surely Fred is referring to the brake failure 'shuttle' switch while Andrew seems to be referring to the brake-light switch. Mal-adjustment of the brake-light switch can cause the brakes to stick on, which would eventually overheat them and cause fade i.e. reduced braking effect, but that would only be after some driving. Like Clifton I cannot really see why removing the shuttle switch should affect braking effort. If there was some dirt and it is now shifted then replacing the switch should not re-affect the brakes, although the shuttle may now be biased to one side hence the warning light will be on.
Paul Hunt

Opps! Sorry Fred, I should have read your post more carefully. I was supposed to be "working" at the time when I dashed off my post.:) I would agree with Paul about the source of the wear on one side of the switch. I have heard of people simply disabling the warning light, knowing full well that the brake system was OK, but unable to make the light go out otherwise. In which case the switch was probably pushed to one side or the other of the shuttle valve, on all the time. The pulsation of the fluid under pressure would then cause wear at that part of the switch over a period of time.
Regards, Andy
Andrew Blackley

Thanks for your advice. I took the switch out and everything is working fine. I watched the shuttle valve and had my wife step hard on the brakes the valve never moved. The switch is the only original part in the brake system. Why is was worn on one side and why that makes a difference I can't figure out, but as long as it is now working I'm not going to complain. The failure light never came on but I know the switch is good, I tested it with a meter. Now it's time to move on and try to setup the Weber carb.
Thanks again,
Fred
FRED DOHERTY

I replaced the o-rings in the shuttle valve of my pressure switch last weekend. While I now have stopped the fluid leak through the switch, the purpose behind this endeavor, the pressure warning light stays on all the time now. I have an excellent pedal and have a switch on order, feeling that all that fluid leaking through the switch has ruined it, although it was never on before I rebuilt the shuttle valve and was leaking. Could it be that I haven't centralized it? My Bentley & Clymer manual only illustrate the older style valves, "pre-o-ring". Do you think a new switch will cancel the warning light? The brakes now feel better than they ever have in the 4 months I have driven the car,even better than after changing pads, shoes and rotors. If any one has instructions how to centralize the shuttle, I would appreciate it. I installed the inner piston assembly the way it was when I disassembled it(the piston looks like a small old fashioned dumb-bell with grooves cut into it for the o-ring)
Joe

Joe -

The plunger on the switch should be normally extended, and you should be able to press it in and it
should return to it's extended position without any hangups.

You might want to try reconnecting the wires on the switch - without screwing it into the valve housing
and work the plunger in-out with your hand to see if this will extinguish the warning light.

While the switch is out - you can look into the hole where it screws into the valve body to see whether
if the piston is centered. You'll probably need a flashlight for this. If the piston is off-center, it can be
gently levered into position with a small screwdriver.
Daniel Wong

To all reading this post. The purpose of the brake failure switch is twofold: 1. The warning light goes on if one half of the braking system fails to warn the driver. 2. More importantly, it holds the brake failure shuttle to one side so that the brake fluid from the "good" part of the brakes does not lose fluid. Normally, the switch sits on the raised section in the centre of the shuttle. BUT, you MUST remove the switch when you bleed the brakes, otherwise the shuttle will not centre and you only get half the braking effort and I assume, a hard pedal. Fred, if you have been braking with the shuttle to one side,that would explain the flattening on one side of the switch plunger. When you removed the switch, the shuttle centered itself and bingo! you had full braking.
Now that you have centered the shuttle, you MUST put the switch back in, otherwise when one of your brake circuits fails, you will eventually loose ALL your fluid. My switch is permanently "on" electrically, so I have disconnected the wires at the switch and I check my master cylinder regularly (as we all do, of course!). These switches are virtually impossible to find new in Oz. All this information came from my local, friendly brake and clutch man, "BGT" (Brian, George and Tom, not as in MGBGT), in Hawthorn, Melbourne, for all you Aussies reading this.
Stay switched on,
Greg, 79B, ex-California, LHD to RHD conversion, everday driver. Love those rubber bumpers!
Greg Taplin

This thread was discussed between 26/04/2002 and 01/05/2002

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