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MG MGB Technical - Master cylinder

I have removed the pedal box from my '73 V8GT, to replace both master cylinders.
Between the M/c and the frame mounting face there is a brass shim. In this case 0.008".
Why is it there, and will the thickness vary, to fulfil its function?
Allan Reeling

It's only listed for the brake M/C, Mk2 on until dual master. It may just be there to level the pedals when they are released, it's how far the M/C piston comes back that determines that. 8 thou might not sound like much until you take into account the leverage of the pedal.

Having said that the original ZS had wildly different clutch and brake pedal heights at rest but it never bothered me.
paulh4

Paul, My V8 is single line, tin can M/c, and the shim was definitely there when I first re-built the cylinder's innards, 14 years ago.
I understand the leverage ratio issue but 0.008" would only equate to something like 0.035" at the business end. I doubt whether anyone would notice that, even if obsessed with heel and toe!!!.
My "mulling" this more is more along the lines of ensuring fluid recovery in the cylinder, bearing in mind that the welded up pedal box and it's pivot drilling, are unlikely to be all that accurate. 🤷🏽‍♂️

"Who knows the secret in the Black Magic pedal Box?"


Allan Reeling

Fluid recovery? You'll have to explain.

I'd agree with the pedal frame not being terribly consistent from one to another, but can't see that shim making any difference to that given that it was apparently on all (Mk2) brake M/Cs and no clutch M/Cs.
paulh4

Fluid recovery....................the piston retracting far enough to allow reservoir fluid to "top up" the system as the friction material wears................it's only a brain constructed theory. But often over adjustment of the stop lamp switch does create a problem when the pedal can't relax far enough.
Allan Reeling

The brake cylinder has a 'slow release' or restrictor valve on it's output. This allows fluid to flow rapidly out of the master but only slowly return from the lines i.e. from the calipers and wheel cylinders. This allows you to 'pump up' the brakes if there is air in the system or the handbrake is very badly adjusted, instead of the pedal going to the floor every time you pumped the pedal. The clutch doesn't have this valve.

The spring in the cylinder pushes the M/C piston and the pedal all the way back as soon as you release the pedal. The restrictor valve creates a negative pressure in front of the pressure seal which pulls fluid from behind it from the reservoir.

With the piston all the way back it opens a bypass port so as fluid slowly comes back from the lines it is pushed back up into the reservoir.

Just like the clutch, as things wear less fluid comes back than was pushed out, replenished from the reservoir when the pedals are released, hence the level drops.

The RB and V8 brake light switch on mine only had 1.5 turns between failing to turn the lights off (not screwed in enough) and stopping the pedal coming all the way back (screwed in too far). In the latter case that can prevent fluid expansion from heat passing up into the reservoir, applying the brakes instead.

The pedal should always come back far enough to operate the switch, if it doesn't there is a problem inside the master or with the pedal pivot or linkage.
paulh4

My theory concerning the shim, is that MG used it to ensure that the piston returned fully to prevent the cup seal either fully or partially obscuring the by-pass hole. Perhaps they had seen a problem and it was a sticking plaster fix/prevention🤷🏽‍♂️
Allan Reeling

If you disconnect the pedal clevis pin the pedal spring should pull the pedals back quite a bit more than their normal resting position. It's the fully returned M/C piston and push-rod hitting the circlip that determines that resting position, not the pedal or anything else external to the M/C hitting a stop.

A defect in the push-rod, clevis pin, pedal pivot or pedal itself preventing the pedal from fully returning could well prevent piston from fully returning, but a defect such as that would be completely random in how much it affected the piston so you would never know what thickness of shim to provide.

If the defect is inside the M/C with the piston or push-rod failing to come back far enough then a shim between M/C and pedal frame wouldn't help.
paulh4

BTW this picture was taken to show how the pedal linkage can wear but it also shows how much further the pedal return spring has caused the pedal to return once the clevis pin has been removed. Clutch, but the same applies to the brake.


paulh4

Well MG/Lockheed placed a shim there for a reason best known to them?
Allan Reeling

Indeed they did, and the reason seems to remain a mystery.

Have you read the MG Experience thread on this? Some interesting suggestions ...
paulh4

Mystery indeed!!!
Allan Reeling

This thread was discussed between 30/08/2026 and 05/09/2026

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