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MG MGA - Master Cylinder Seal Change

I had an interesting conversation with a motor mechanic the other day. He said whenever you change seals in brake and clutch cylinders you should always first lightly hone the bores to 'break the glaze'. This will allow the new seals to bed in better and more quickly and probably slide better in the first instance.

It's a new one on me but happy to learn. Any comments?

Steve
Steve Gyles

I don't believe it. For piston rings, ok not soft elastomers.
ArtPearse

If the bore is scratched or pitted it has to be honed until smooth (if that is possible at all). If you can't hone out the defects it is a lost cause and must be sleeved. If honed it has to be finished with a fine grit stone for very smooth surface finish to allow the seals to actually seal.

If it appears to be smooth enough to work without honing, then try new seals first. If it seals okay without honing you win. If it leaks you get to do it over again (and maybe fix the paint as well if it as not silicone fluid).
Barney Gaylord

Glaze is your friend - specially if it's filling the irregularities in the bottom of the bore. Of course, if you're ready and able to spring the cash for a re-sleeve, go for it, but otherwise, looking for pits isn't necessary.

Maybe I've been unlucky, but I haven't yet come across pits that were shallow enough to hone out.
Nick and Cherry Scoop

Interesting feed back. Thank you all.

The saying goes never to declare your full hand straight off. My conversation with the mechanic came about because I noticed (seat of the pants feel once again) on my run back from Brooklands last Sunday that the brakes were slow to fully release.

Analysis quickly pointed to the MC (releasing a bleed nipple freed it all up instantly). I noticed also that the MC brake piston was slow to fully retract. Checking the MC out this morning I discovered that the small forward hole in the brake cylinder casting had reduced in size. The pointed end of a needle just made it through to the inner lining. I ran a small drill through which immediately cut through 'some wall lining', (no metal; I did not enlargen the hole in that way). I also cleaned the cylinder walls with my favoured stainless steel metal polish and, after a thorough wash through, all has returned to excellent working order.

I am a strong supporter of the benefits of silicon fluid, but it does make me wonder if this fluid is susceptible to a 'lining' build-up on cylinder walls and holes over a period of time. It was certainly the opinion of the mechanic.

Steve
Steve Gyles

An odd thought occurred to me while I was on my morning walk to get the paper this morning. I do not recall 1500 owners with drum brakes complaining about brake slow release, squeal, etc; but there again I may be wrong.

With the introduction of the 1600 and front disc brakes the MC was modified with the extra fluid capacity cover. However, I just wonder if Lockheed also slightly modified the inside with a larger forward return hole? My experience with the 1500 disc conversion has always been marginal with brake release, despite the absolute knowledge that the subject hole is not blocked by the main seal. It has also been a similar experience on 3 MCs to date.

I just wonder if that hole, the diameter of which was originally calibrated for drums, needs to be just a little larger for discs? With drum brakes you only need a marginal pressure release for the drum linings to retract in overall diameter to fully release the brakes with no surface contact, whereas discs remain in contact to some degree throughout their operating range.

Just a thought. Probably rubbish.

Steve
Steve Gyles

Steve

I have the same MC as you, and MGA discs brakes up front. I have never had any problems with the disc brakes at all, even with two different MCs in the last 26 years.

I only changed the MC because it was twenty years old and I figured that was probably coming to the end of its life.
dominic clancy

If brakes are slow to release or drag, check to see if the hoses are collapsed inside restricting return flow.

If brakes drag and opening a bleed nipple frees them up, then also check the hose.

If disc brakes drag, and they still drag after opening the bleed nipple, then you need to replace the piston seals. The disc brake piston seals have "resiliency" (flexibility of the rubber) that will retract the pistons slightly when pressure is released. Disc brakes in good condition do not drag, they run a few thousandths clearance for the brake pads when released.
Barney Gaylord

Based on Barney's comments I've decided to take the precaution of overhauling my MGB calipers and change the flexible brake hoses.

During a bit of research on the topic I came across numerous MGB forums where they have changed their standard MGB calipers for the Austin Princess 4 pot calipers which are, apparently, a straight swap. The stopping power is reported as excellent in comparison. Has anyone gone this route with their MGA and, if so, what were the findings?

Steve
Steve Gyles

AFAIK the B and A callipers have different mountings, will the Princess callipers fit a standard MGA disc brake mounting?
dominic clancy

Dominic

Apparently they fit the B caliper bracket which is what I have in the car to accept the B caliper. Moss market the bracket as part of their MGA disc conversion kit.

Since my last post I have searched the archive. Terry Drinkwater uses the AP 4 pot in his race car and says they are very powerful.

Steve
Steve Gyles

Steve,
What s the weight difference between the A/B callipers and the 4 pot Austin Princess callipers?

Mark
M Wellard

Mark

I can't answer that one. I only have the B caliper.

It seems the AP caliper is very popular with other 'race' and fast road cars. Apparently also used by some Ford Escort owners. The Austin Princess is looked down on by most enthusiasts, but its brakes were supposedly excellent. There are reports of people buying scrap AP cars for about £100 just to take the brakes off! They are now becoming like the proverbial rocking horse s..t. The AP owners' club will not sell brake parts to non owners according to some reports.

Steve
Steve Gyles

This thread was discussed between 15/04/2015 and 03/05/2015

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