It's likely that when the OD was installed the mechanic used Dot3 in the master cylinder, the same as he would a Chevy or Ford. It doesn't take long at all for the incorrect fluid to attack the unique natural rubber seals used in all British Leyland cars and turn them to goo.
This is evidenced by looking at the fluid in the reservoir and seeing if it's clear. If it's cloudy black, that's the rubber being dissolved into the Dot3 fluid. Only Dot4 is acceptable in your car. Not Dot5 or silicone mind you, but regular Castrol Dot4 brake fluid available in most any parts store.
The same applies for the brake system. If you added any Dot3 to the brake master cylinder, get it out now before it gets to the four wheel cylinders, costing you even more money. If you have access to a fluid pump (they're about $15), suck the remnants of the contaminated fluid out and bleed the brakes anew after the Dot3 is completely purged from the system.
As I mentioned, a rebuild is pretty straightforward and easy enough for a backyard mechanic. It involves only removing the piston, cleaning up scores with emery cloth, honing the cylinder with a cheap hone attached to a drill, and replacing the rubber seals included in the kit. The unnecessary cost of new hydraulic cylinders for British cars can cause sticker shock on a par to a heart attack.
A good friend of mine owns a great local auto recycling shop here in NE Ohio. Team Triumph specializes in hard to find British car parts. His warehouse has a garage door at one end, and as a junk Triumph or MG proceeds down the disassembly line, it disappears into the racks and is gone completely by the time it reaches the end. His extensive supply vintage inventory is legendary among the community, with customers as far away as Europe and Australia.
Scott Harper has been doing this for almost 25 years, and he is a walking encyclopedia of British car knowledge. When you call there, it's Scott himself who answers the phone (tell him I said hi). He and his right hand man Denny also get their hands dirty tearing down cars, cleaning parts, and packing parts for the UPS truck that arrives twice a day. He's very honest, fair, and he discounts most Moss motors parts 10%, possibly including your rubber seal kits for the master & slave cylinders. As a plus, he's a devout liberal democrat whose daughter works for the national party.
I strongly suggest you call Scott for all your future MGB parts needs. Here's his website with all pertinent contact info:
http://www.teamtriumph.com/
Good luck, and let me know how your project goes.
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