Author Topic: U12 2.0 Exec brake master cylinder  (Read 173 times)

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Offline Mister_T

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U12 2.0 Exec brake master cylinder
« on: Jul 18, 2020, 05:53PM »
Hi people,
My ol' 50-Shades-Of-Brown Pinty was stolen and recovered so now I'm looking at how much it will cost to fix it and whether it would be worth it.
There is a buckled front rim and a wheel alignment needed at very least.

While I've had the car stored at work, the master cylinder emptied somehow. I topped up the fluid but still get no pedal. Heck, I drove from the holding yard back to work and it was ok.
For now I'm guessing the master cylinder is leaking into the booster. If one rear slave or one front caliper is leaking I assume the twin circuit brake design would still give me some sort of braking and resistance in the pedal. It may be a while before I can get a more thorough look into it.

So I look around eBay for a replacement master cylinder. Even if the master cylinder is not the actual problem, it probably deserves to be replaced after 29 years anyway.
It seems that the masters are not so easy to find. There is one eBay seller of a Protex master and claims it fits both U12 and R31 but the picture shown does not give me confidence. The bore is quoted as 22.22mm, Corsair FSM says it should be 23.81mm. Also, one port for a pipe is 90deg off.
For the curious:

Darn, can't even find a master cylinder kit on eBay.

So here is a question: Is the drum-rear-brake master cylinder the same as the all-disc models?
The Corsair FSM makes no mention of drum rears.

Perhaps another question: Even if the bore size is the same for both types of rear brake, does the dual proportioning valve inside the master cylinder have the same split point? The Gregorys manual only lists one bore size in its specs (23.81mm) but makes no mention of the proportioning valve split point.