Master cylinder rebuild question

Hi guys - question on my troubled Master Cylinder rebuild. Once assembled but off the car the brake piston can be pushed back into the cylinder and it will return on its own. The clutch piston however gets stuck once you push it into the cylinder. I know on the car with a bleed system there will be some force helping the spring to return but this just doesn't seem right.

The piston is supposed to return to the opening by itself, correct? There is a little bit of brake fluid in the bore so it isn't like I am trying this dry or anything.

Ideas? Coaching?

OK - did a bit of Googling and found the following. MGA site - Barney's actually but it applies;

"A common problem with master cylinder rebuild is that the piston(s) may stick in the bore(s) and not return. This is usually not a problem with the clutch, as the clutch provides a substantial return pressure to help return the master cylinder piston. <span style="font-weight: bold">The issue may be with "modern" replacement seals being a bit too large for the bore</span>.

The fix or cure is best carried out on the work bench immediately after the master cylinder rebuild and before installing it in the car. If the master cylinder piston does not return when pushed in, then apply air pressure to the output port to return the piston to normal rest position. Push the input pushrod again to depress the piston full stroke and release. Use air pressure to return it again if necessary. Repeat the process as many times as required until the piston will return freely from the internal spring force only. This may require dozens of cycles of depression and air return to get it to return freely with the spring force only."

https://mgaguru.com/mgtech/hydraulics/ht204a.htm

OK - so I'm not completely crazy then...

Where'd ya get th' kits?

...I'd be for an attempt at bench bleeding the thing before ya condemn it.

Hi Doc. Moss, and I'll follow Barney's coaching before I condemn it.

Just afraid of having to tell Jack she's not running yet. Grin.

tdskip said:

<span style="font-weight: bold">The issue may be with "modern" replacement seals being a bit too large for the bore</span>.

OK - so I'm not completely crazy then...

Click to expand...

Yep, this is a known issue. The manufacturer of the most commonly available seals are sometimes out of tolerance, mostly too big, but had one too small in a Morris last summer. The rubber is harder too, so sometimes that contributes. If the suggestion doesn't work, you'll need another set.

Thanks Gerard - I worked on moving the piston back and forth and turning it in circles in the bore last night and it now returns fine. I'm going to double check the brake side to make sure that one travels freely.

Hope this thread helps someone else before they get the MC back onto the car. apologies if everyone but me already knew this.

Well, everyone didn't know. Thanks for the heads up.

After a full rebuild it seems lots of things work better with some use.

Engines, seals, etc.

Hi guys - a follow on here. So I got the brake piston moving freely while the MC was off the car. Installed and sure enough the brake piston got hung up again. I was not a happy camper. Had to pull everything again. Fun times...

Got it working off the car again, back on again and this time I have brakes. And clutch. So she goes and stops Jack, just took a bit longer than expected.

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).

Excellent.

Just out of curiosity what are people using to clean up the bores on aluminum master and wheel cylinders?

Do any of these have aluminum cylinders? I was of the impression that all Spridgets had cast iron wheel and master cylinders. In fact, I don't know of any of the ordinary British cars that have aluminum cylinders.

TR-2’s, TR-3”s and TR-4s have aluminum master cylinders. But no Spridgets, In the stock configuration that is!

Sarastro said:

Do any of these have aluminum cylinders? I was of the impression that all Spridgets had cast iron wheel and master cylinders. In fact, I don't know of any of the ordinary British cars that have aluminum cylinders.

Click to expand...

Early cars (Bugeyes and Spridgets) before disc brakes were standard have aluminum, single piston rear wheel cylinders. Same with all Morris Minors.