MGA / Midget Master Cylinders

Questions, answers and reports on various technical subjects.
tdskip
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Re: MGA / Midget Master Cylinders

Post by tdskip »

Steve - you may want to make sure to really exercise the pistons/cups before bolting everything back up. I got some time to wrench on the Bugeye yesterday and I got the brake piston moving freely while the MC was off the car. Installed the MC and sure enough the brake piston got hung up again. I was not a happy camper. Had to pull everything again. Fun times...

I also took a piece of MIG wire and cleaned the small holes in the MC as one of them was blocked.

Got it working off the car again, back on again and this time I have brakes. And clutch. In the back of my head I am expecting it to be sticking again by the weekend however (she is at the hanger so I won't be be able to driver her again for a couple of days).

Anyway, my take away is that the pistons need to move very freely before assuming that any stickiness will be overcome by the brake/clutch lines being bleed.
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Re: MGA / Midget Master Cylinders

Post by Steve Simmons »

One cause of stickiness is the secondary seal appearing to be seated, but not actually be fully in the groove. Sometimes you have to really work it in there, pushing back and down around the circumference of the assembly. I had this problem on a TC MC and actually had to machine the piston groove wider to accommodate the modern seals which are slightly thicker and less compliant than the old ones.

If I do have to pull the MC again, at least I've become a certified expert by now. I can get the thing out in about 5 minutes.

Nate, if I charged half the going rate, I'd still charge too much because everything takes so darn long! :roll:
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Re: MGA / Midget Master Cylinders

Post by tdskip »

Steve Simmons wrote: Nate, if I charged half the going rate, I'd still charge too much because everything takes so darn long! :roll:
No kidding - I've actually had the thought that it would be cheaper overall to just buy a new one and treat them as disposable. I hate that thought, but I've lost +/=6 hours screwing around with this which has a high opportunity cost on other stuff I could be doing.
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Re: MGA / Midget Master Cylinders

Post by Larry Kluss »

Good work, Steve. You bring up a good point about replacement parts and a lesson for anyone rebuilding their M/C's in the future. ALWAYS compare your new parts to the old ones. If the seal or piston is different (longer, shorter, bigger, smaller, etc.), be aware that it may not work as it should.

Now as far as sticking, the m/c piston should ALWAYS return easily on the bench (assuming it was properly lubed). A light coating of brake fluid on the seals before installing them on the piston and then a light coating in the bore before installing the piston, is sufficient. If the piston doesn't move freely, there is a problem, period.

You mentioned having to work the seal into the groove. That simply is not right. The seal is too long for the groove and/or the groove too narrow for the seal. If it's too tight in the width of the groove, the back end can bunch up (or stick out diametrically) and make the piston stick upon return.

I should have thought to ask you that sooner. :hammer:
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Re: MGA / Midget Master Cylinders

Post by Steve Simmons »

The groove thing was on a TC MC. The MGA one seems ok. I was sure to work it in there anyway, as it has become habit after the h*ll I went through with the TC brakes last year. That saga was twice as traumatic as this one.
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Re: MGA / Midget Master Cylinders

Post by Larry Kluss »

Okay, so where exactly do things stand now?
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Re: MGA / Midget Master Cylinders

Post by Steve Simmons »

MC is rebuilt, and I'm about to bench bleed and reinstall. I should be to the brake bleeding stage in an hour or two.

Another note on the rubber cups, I noticed the longer ones are also much stiffer than the shorter ones I just got and the finish on the rubber is a little different. The markings are all exactly the same. I'm running the shorter, softer one now on the brake side. I accidentally put the stiffer one on the clutch side but I'm not doing it over. Even when it was super sticky before, the clutch worked fine, probably because of the pressure plate forcing it home.

EDIT: There's another problem somewhere. I put the pressure bleeder on it with 10 PSI in the line. I get only a dribble from the bleeders. Tried one front and one rear. WTF?!? :?

Hoses are new, and the brake seemed to work perfectly on the bench - lots of fluid when pumped. Guess I have to start cracking hard lines to see if there's a blockage somewhere.
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Re: MGA / Midget Master Cylinders

Post by tdskip »

Ah bugger!

What a pain Steve, sorry to hear it.
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Re: MGA / Midget Master Cylinders

Post by Steve Simmons »

Cracked open the main pipe at the union, had the wife unit push the pedal and fluid came out at warp speed. With the pressure bleeder applied, it doesn't come out as quickly.

In other news I have no skin left on my fingertips or forearms.
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Re: MGA / Midget Master Cylinders

Post by tdskip »

Ah - so everything was OK and it was just the difference in relative force applied to the fluid?
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Re: MGA / Midget Master Cylinders

Post by Steve Simmons »

I dunno, I still can't get them bled!
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Re: MGA / Midget Master Cylinders

Post by tdskip »

Are you getting flow with a person pushing on the pedal at the bleed nipples?
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Re: MGA / Midget Master Cylinders

Post by Steve Simmons »

Yeah, I'm getting flow, maybe not as much as I'd like but it's flowing. No more air coming out and still a soft pedal.

At least it's back to the point that I can pump the pedal and get some pressure. But, while the piston snapped back hard and fast on the bench, now it's slow to return. It's acting like there's a vacuum working against the piston return spring.
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Re: MGA / Midget Master Cylinders

Post by Larry Kluss »

Steve, I doubt there is a line blockage. 10 PSI is not much so the small about of fluid movement seems okay to me. Unless you have another car to compare the same 10 psi test.

A "vacuum" or pull back sounds like the pressure cup seal is not valving correctly. upon release, the majority of the fluid goes through the larger reservoir hole and (in your case) through the small axial holes in the piston and past the cup seal.

I assume the thin metal washer is in it's proper place behind the cup seal? Is the washer new? If so, how does it compare to the original one?
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Re: MGA / Midget Master Cylinders

Post by tdskip »

Steve - same symptom as I had. Had to work the seals more.
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Re: MGA / Midget Master Cylinders

Post by tdskip »

Any update Steve?
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Re: MGA / Midget Master Cylinders

Post by Steve Simmons »

Semi success! Thanks to Larry's assistance and impressive vacuum bleeder, we have the brakes working safely. They still return slowly and there is still friction when pressing them down, but we have an acceptably firm pedal and the brakes will lock up if jammed on hard. So, the MGA is now safe to drive on the road. I'll drive it around for a while and see if the MC smooths out, and eventually bleed once or twice more.

Thanks again Larry, you rock! :)
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Re: MGA / Midget Master Cylinders

Post by Larry Kluss »

Glad I could be of help. You were most of the way there. Just needed a helping hand and a good tool to get over the hump.

I enjoyed seeing your soon-to-be previous residence.
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Re: MGA / Midget Master Cylinders

Post by tdskip »

How slow is the pedal to return Steve? I've got the same behavior on the Bugeye (same seals and MC) and it is OK on predictable roads where I can brake once ahead of time, traffic lights and etc. It will lock the brakes on a single firm application BUT for twisty roads where you have a quick one / two stab at the pedal I'm not sure it would rebound fast enough...
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Re: MGA / Midget Master Cylinders

Post by Steve Simmons »

It doesn't hold pressure, it just returns slow. So if I had to hit them again right away then the pedal would go down without any resistance until the plunger hit the piston again. It is not dangerous by any means, just a bit strange feeling. My brakes are adjusted so tight that they only travel about 10-15% before the car begins slowing, so the piston doesn't have far to return anyway.

I'm hoping that the rubber and bores will all loosen up a bit over time. I still regret not having the bores polished. I know that has cured the problem for some people. For now, I don't plan to touch it because I'm thrilled to be able to drive the A with confidence again!
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Re: MGA / Midget Master Cylinders

Post by tdskip »

Cool!

Tight at the drums or push rod against the piston?
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Steve Simmons
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Re: MGA / Midget Master Cylinders

Post by Steve Simmons »

At the drums. You definitely don't want to run the push rod too tight against the piston. There MUST be play there.
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