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MG MGB Technical - Binding brakes

Despite changing calipers and wheel cylinders I still feel the brakes are a bit sticky now and again. I read recently that this might be due to a sticking piston in the servo. Is this likely and if so how do I get to the piston on a 1975 mgbgt; do I remove the ring of screws on the white cap?
R E Merrall

Does your UK spec B use a hydraulically operated brake light switch or a mechanically operated one? If it is the latter, it may simply be tightened too much and is not allowing the brake pedal to return to its fully retracted position which will cause the brakes to stay partially engaged. RAY
rjm RAY

Ray

To be honest I don't know. The switch is on the end of the master cylinder.

Rod
R E Merrall

Rod,
Read this from Paul hunt before messing with the servo.
http://www.mgb-stuff.org.uk/braketext.htm#mcvalve
There are a few things that could cause occational sticking.
Best of....
MGmike
M McAndrew

With a 75 i.e. rubber bumper I'd expect to see the switch screwed in to the pedal cover, and as Ray says if this is screwed in too far it prevents the master piston coming back far enough which can cause the brakes to progressively lock on harder as the brakes and hence the fluid heats up. There should be a little bit of free play in the pedal, and in my experience there are only 1.5 turns of the switch between just going off and reducing the pedal free play, so it is quite critical, see here and scroll down to the section on rubber bumpers.

Other than that there can be other causes like one of the hoses breaking down internally and preventing fluid flowing back. If it's a front hose then only that wheel will be affected i.e. getting hot, if the rear hose then only the rear brakes will be getting hot.

It could be the valve referenced in the link given by MGMike, although I think that less likely.

If it's the servo or the mechanical brake light switch then all four wheels will get hot. You can eliminate the remote servo by temporarily disconnecting the hose from it and plugging the hose, as the servo makes little difference in my experience trying that on several cars, but from what others have said the integral servo (77 and later in the UK) the servo makes a bigger difference and disconnecting it may make a significant difference to braking.
Paul Hunt

that's what I expected too Paul but the switch is not on the brake pedal, it is on the master cylinder - see photo. If I rotate this switch outwards (anticlockwise) the brake lights eventually stay on. Do you know how to correctly adjust the switch? Picture to follow.
R E Merrall

Can't get the upload to work, try later
R E Merrall

Photo attached

R E Merrall

Your brake switch is mounted on the pedal box cover that is mounted above the brake and clutch master cylinders. The master cylinders are actually mounted backwards, so it would appear that the switch is actuated by the master cylinder itself. The brake master cylinder pedal is what actually comes into contact with the brake light switch. When you depress the brake pedal, the front side of the pedal moves away from the brake switch actuating the brake lights. Inside the brake switch is a pair of electrical contacts that are bridged when the internal spring loaded plunger moves away from the switch body and turns on the brake lights. Rotating the switch, relative to the pedal box, moves the plunger closer or farther away from the point where the brake lights come on. RAY
rjm RAY

Rod,It sounds like you need to adjust your brake pedal to give you the correct free movement as detailed in the workshop manual.

See the attached which may help.

Andy

Andy Robinson

Thanks for the explanation and diagram; I'll check it out
R E Merrall

Rod, when you say "sticky" do you mean they tend to hang on when you have released the pedal?
Allan Reeling

As Ray says, the switch may well be mounted on the cover but it is the pedal that actuates the plunger. Also you don't adjust the pedal, you adjust the switch to give the correct free play of the pedal which hopefully will give enough movement of the switch to turn the lights off reliably. If the switch is screwed in too far it stops the pedal coming back as far as it should, which prevents the piston and its seal coming back far enough to clear the bypass hole to the reservoir. Unless that hole is clear fluid expansion from heat from using the brakes causes the brakes to be applied with the pedal fully released.

As I said earlier it only takes about 1 1/2 turns in my experience between the brake lights just going off and restricting the free play in the pedal. I implied there was a link on how to adjust but forgot to include it, it is here http://www.mgb-stuff.org.uk/electricstext2.htm#blswitch, scroll down to the RB section. It is basically a matter of unscrewing the switch until the brake lights stay on, then screwing in carefully until they just go off, then screw in further, counting turns until you start to reduce the pedal free play below 1/8" at the foot pad. If that's more than half a turn you should be OK.
Paul Hunt

Thanks for the advice; yes Allan that's exactly what they do but not all the time. Drove about 130 miles today with no bother.
R E Merrall

Had a look at the brake pedal but there is no means of adjustment there see photo. The second photo is looking up where the brake pedal goes up I assume to the master cylinder

R E Merrall

I'st photo attached

R E Merrall

Rod, the adjustment of free movement on the brake pedal and the brake light switch is achieved by adjusting the position of the brake light switch with the pedal box cover in situ.

As Paul has said " Also you don't adjust the pedal, you adjust the switch to give the correct free play of the pedal which hopefully will give enough movement of the switch to turn the lights off reliably. If the switch is screwed in too far it stops the pedal coming back as far as it should, which prevents the piston and its seal coming back far enough to clear the bypass hole to the reservoir."

It sounds like you need to unscrew the brake light switch to achieve the correct clearance and stop the brakes binding.

Andy
Andy Robinson

I understand at last.Bit slow these days. Thanks to all
R E Merrall

Glad that you figured it out. Doing things, like this, can often appear puzzling at first. RAY
rjm RAY

This thread was discussed between 23/09/2015 and 30/09/2015

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