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MG MGA - No hydraulic 'prime' on clutch side of MC

I just replaced the MC in the MG. All of the brakes are bled and brake pedal feel is nice and tight! I used my pressure bleeder on the brake circuit, and attempted the same on the clutch side without success. This is where I need some help...

I do not seem to be able to get a hydraulic 'prime' on the clutch side. That is, no amount of pedal pumping is moving fluid through the clutch hydraulic line. I have removed the clutch slave cylinder and am now just trying to get fluid to flow from the open end of the hose...almost nothing. If the car sits awhile (an hour or two/ or overnight) I get flow through about 1/4 of the pedal travel (top of stroke), then nothing. Repeat pumps...nothing additional. To state the obvious, I am made sure that the fluid level in the MC is full.

I have read the variety of great advice for removing the last bit of air including by reverse bleeding, that is, pushing fluid up from the bottom. The behavior is like I have brake fluid on only one side of the MC piston. I have never encountered this situation before, and am unsure how to proceed.

Any thoughts?

Rick
rpb bunch

Rick, it sounds like something is restricting the passage of fluid from the reservoir to the cylinder, could the feed be obstructed by the clutch M/C seal? Is the piston restoring fully, is there enough clearance on the clutch pedal push rod? Failing that, a piece of crud in the fluid feed hole or incomplete drilling?
Lindsay Sampford

Thanks, Lindsey...

I'll try to remove/readjust the push rod in the morning. If that does not do it, then step 2 will be to remove the front cover to visually inspect the clutch piston resting position and travel. Per Barney's site, I may have a sticky piston. I'll let you know how I get on.

Rick
rpb bunch

This morning I shortened the clutch pedal push rod by about 1/4". No change in behavior...still no hydraulic pressure. I still have a little more that I can shorten it (threaded adjustment) but my sense is that this is not the problem. I think I'm looking at a stuck piston on the clutch side at the furthest point of the stroke. Disappointing, because it's a brand new MC!

This evening I will disassemble the clevis to the pedal, and see how much travel I have in the push rod before I touch the piston.

Stand by...


rpb bunch

'How I spent my Saturday morning...'

Today I disconnected both clevis links. The brake side pushes in by hand, and returns (slowly) under return spring force. The clutch side pushes in, then stays in indefinately. If I apply air pressure from the bottom, I can get the clutch side piston to return 'home', albeit with a distinct 'snap'. If I push manually again...same behavior.

I rebuilt and bench tested a used MC this morning. Both the brake and clutch cylinders return under the return spring force. I re-installed the rebuilt MC, pulled out the new MC, and am back on the road. I will be returning the new MC this week. Post mortem bench testing of the new MC behaves the same...I can push in the clutch push rod, and it stays compressed. The brake side pushes in and returns under the return spring force.
rpb bunch

Rick, out of curiosity, what type of hydraulic fluid were you using?
Lindsay Sampford

Lindsay...

I use DOT-4, glycol-based, (sourced from NAPA)in the MG and an old Jeep CJ that I run. Our two modern cars, which are daily drivers, are filled with silicone brake fluid.

Cheers,

Rick
rpb bunch

Thank goodness for that Rick!
Lindsay Sampford

I had the similar problem with a stuck piston during a M/C rebuild a few years ago. Problem turned out to be operator error (on my part) in not getting the secondary piston seal properly seated on the piston. It didn't take much for the piston get stuck in the bore. On reseating the seal, the M/C performed correctly. Hard to believe that it could happen with a new unit?

Glad that you are back on the road!
Don
Don Carlberg

Don raises an interesting point about this sort of problem with a new MC. There is some rubbish being manufactured nowadays.

Who made/supplied it?

Steve
Steve Gyles

Rick,

After I rebuilt my M/C I had a bit of trouble getting the clutch side to create pressure, however much I pumped the pedal. However, once I set up my Eezibleed and forced fluid through to the slave cylinder it started to behave normally. This was in an original, not after-market M/C in which I had just honed the bore to clean it up. I suspect it takes a while for the new rubber seal to slide easily in the bore and by forcing the fluid in I created the conditions it needed to move more easily with the spring pressure.

Regards

Malcolm
Malcolm Eades

I know it does not help Rick as he as already bought a new MC, but I commend the AP Caparo Lockheed new-build MC. Looks exactly the same as the original (other than some external lettering) as the company uses the Lockheed tooling. However, the internal machining is to much higher quality and tolerance than Lockheed achieved in recent years. I noticed the difference immediately. Yes, they cost nearly double some of the MCs you see advertised on Ebay, but in my opinion worth every penny at about £160 to £180 ($270) - Don't quote me on price as I do not have the receipt at hand.

For those in the UK speak to Bob West. He has them in stock. There are almost certainly other suppliers.

This guy at Gerards Garage seem to say the same thing: http://www.gerardsgarage.com/Garage/Tech/DBrake/TandemMC_Part1e.htm

Steve
Steve Gyles

This thread was discussed between 09/09/2010 and 13/09/2010

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