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MG MGB Technical - Pedal Box Query

I recently rebuilt my calipers and replaced the flexible hoses, as they hadn't been done for over 20 years.

After bleeding the brakes with my Eezibleed, I noticed some fluid on the mat in the driver's footwell.

I removed the cover from the pedal box to find that the master cylinder had developed a leak and, with the help of the Eezibleed, had sprayed fluid inside the pedal box cover. This had stripped the paint off the pedal box and bulkhead. The cover survived unscathed, as it was powder-coated.

As I was having some more parts powder-coated, I decided to have a spare pedal box done at the same time. I picked up the parts yesterday and noticed that the pedal box is different to the original. The hole for the brake m/c is larger and it is missing the reinforcement.

I've just looked at the Moss 'site and it lists different pedal boxes for 3-sync and 4-sync.

I was just wondering why the difference? Were the 4-synchro cars fitted with a much bigger master cylinder, or was it to do with the North American dual-circuit system, perhaps?



Dave O'Neill 2

I know some people have had problems replacing one of the masters - too big for the hole, but that's a big difference to deal with that.

North American Mk2s got a dual circuit master, but that's the only difference I'm aware of, although the clutch master was canted over at the same time to clear the dual brake reservoir cap, found on all cars. All cars got the boosted brake master at various times but the pedal box for those is completely different.

The Parts catalogue doesn't list one for Mk1 cars, only Mk2 on AHH8421, and that changed for RB cars. Several suppliers list that plus AHH6065 for 3-synch i.e. Mk2, but both seem to be NLA anyway.

The same master is shown for both Mk1 and Mk2, with a change during Mk2 at an unknown point which from other research seems to have been an internal change to piston and seal. Externally it's just two concentric rings, although even that is subject to errors and confusion as to shape and position.

Several used ones around, large hole no lower reinforcing as well as same size holes with lower reinforcing, with some indicating the former are Mk1 and the latter just 'CB'. I vaguely remember something about something being strengthened as it distorted under very heavy pedal pressure.

As long as the holes have the same spacing it shouldn't be a problem (and even that's fixable)
paulh4

Just looked at mine and they are both the other way round i.e. a 73 and a 75 with canted clutch master, large brake master hole, and no lower reinforcing! That probably means the larger hole is for the dual master.


paulh4

This thread was discussed on 05/11/2019

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