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MG MGA - Master cylinder questions

I am in the process of refurbishing my m/c. The bores are very good past the reservoir holes and I'm using a Scotch nylon pad with oil to polish them up.There is some pitting in the area between the opening and the reservoir hole, which I think I can hone out if I persevere long enough. My first question is, how important is this area, as it is between the two seals on the piston when at rest.
The second question relates to the position of the reservoir holes. If I insert the piston, with the front front flush with the front of the m/c, then the piston just about covers the hole. With the seal installed it will be past (deeper than) the hole, so how does the fluid from the reservoir get into the business side of the seals?
Art Pearse

Hi, Art...

I in the middle of an MC rebuild too. Keep in mind that on the brake side bore of the cylinder, there is a rubber disc that gets inserted to the back of the bore. The brake piston will stop against this disc, not the rear of the bore.

I, too, am curious about your pitting question, as my MC has pitting opposite the passage into each bore from the reservoir. It seems like this remains between the two piston seals. I'm going to assemble mine and bench test it this way for pressure and leaks.

Rick
rpb bunch

Piston never gets anywhere near back of bore. Pedal hits the floor first.
Barney Gaylord

Barney, the back (bottom) of the bore is OK. Our question is around the front of the bore. With the brake off, the piston is forward. But in the fwd position, the seal cup is covering the hole to the rervoir, so how does the cylinder get equalized with the res.?
Rick, yes, I know about the brake side disc. Thanks.
Art Pearse

Are there two holes each bore connecting to the reservoir? I can see one hole each, about 3/16 dia, and from the reservoir side, about 3/8" furtheown , there is another hole about 1/16 dia, but it does not seem to be drilled through. Is this a tiny orifice in fact? This would be deeper in than the master seal with the brakes off, and would explain my question.
Art Pearse

Found this on Gerard's rebuild web. Shows the two ports per bore.


Art Pearse

Art,

You don't say how badly the MC bores are pitted. Unless the pits are quite shallow, I would suggest having the MC sleeved or get a new one. Any pitting can tear up the seals and/or cause leaks (so much for any new paint) and why take a chance with something as important as brakes.

I think my resleeve job ran about $150 USD. Brass or SS sleeves have the added benefits of not pitting in the future. My sleeves are brass and I am running DOT 5.0 fluid, no leaks after three years. I do seem to recall someone on the board saying that brass MC sleeves and silicone is not a good combination, but so far no problems.

Cheers, John
jjb Backman

John, I am inclined to agree with you. The pit is resisting being honed out and don't want to taper the bore in that area. he pit is in the area of the secondary seal so it could cause leakage. I see Apple Hydraulics do a double sleeve for $140. I think they have a good reputation?
Art Pearse

Art,

I used Apple for a carb rebuild and wasn't totally satisfied with the work. The throttle stops were set at the wrong angle in relation to the throttle disks. I barely have enough throttle screw adjustment to get the idle down far enough. Search the threads for other observations.

I used Sierra Specialty Automotive in Quincy, CA. http://brakecylinder.com/ I was pleased with their work and they are cheaper to boot.

John
jjb Backman

It's already gone to Apple. I'll report back!
Art Pearse

new MCs are also very cheap $209 from Moss USA (so probably less from the dealers that offer 10 or 15% from MOss prices as standard), about the same from the local UK suppliers. For brakes, New has to be better than refurbished, even if it costs more - and with prices now so good, the difference can't be that great when you include an extra shipping charge for a refurb. $209 minus 15% minus $15 shipping comes out at $163

For a $15-25 difference I am personally amazed that anyone even contemplates a refurb any more, especially when the experiences are far from unanimously favourable ...
dominic clancy

Brown & Gammons supplied and fitted a new MC for my Coupe this April the price from memory was around £80-£90 (supply only)
Paul
P D Camp

Dominic, you can rebuild your own master cylinder for under $100, far cheaper than a new reproduction! I have the MC sleeved in brass, and do the rubber bits myself. If there are no machine shops in a person's area who can do the job then that's another story of course...
Steve S

Unless you can be ABSOLUTELY certain of the engineering quality of the company/person that does a sleeve I would always side with Dominic on this one.

That said, having previously fitted a new cheapo MC, I am not convinced that is the way to go either. As I said in another related thread I went down the Rolls Royce route for a Caparo unit (about double the price). Its quality leaves all others in the dustbin.

Steve
Steve Gyles

At least it comes with a brass sleeve and won't rust anymore!
Art Pearse

This thread was discussed between 10/09/2010 and 15/09/2010

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.