MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Clutch hydraulic system won't back-bleed

Through a very long and unsuccessful series of attemptssto get the clutch working in my MGA coupe, I Think I’ve read everything written on MGA hydraulics, including many comments that the clutch hydraulic system can be bled back (upward towards Mstr Cyl) as well as forward (downward from Mstr Cyl).

My system will not back-bleed, however. To try to pinpoint the problem, here’s what I’ve done:

Set-up:

Silicone Fluid.

New 7/8-inch AP Caparo master cylinder from Moss. Bench-bled.

Metal line from Mstr Cyl. is clear. It was removed, alcohol-flushed & blown through.

Mstr Cyl push rods adjusted to give about 2 mm free movement before engaging. Pistons move smoothly in their bores.

Clutch pedal in ‘at rest’ position.

Actions:

Disconnected the fitting that joins the lower end of the metal clutch pipe to the flexible hose. Upper end remains connected to Mstr Cyl.

Ran a clear tube from the lower pipe end into a jar.

Attached a pressure bleeder @15 psi. to Mstr Cyl. A few bubbles at first, but they soon disappear. Bubble-free fluid flows rapidly and smoothly into the jar. 500 ml (18 oz) fluid quickly run through.

Then, to check upward flow:
Remove some fluid from Mstr Cly via the cap opening to make room
Remove the clear tubing from the lower end of the metal clutch pipe.
Attach the pressure bleeder to the lower end of the metal clutch pipe and pump up to 20 psi.
No fluid or air comes back up into the Mstr Cyl., even when left under 20 psi for 15 minutes.

What is wrong?
M.E. Whalen

When you get it to run clear of bubbles in your earlier description hook it up to the slave cylinder, remove the slave cylinder from the car, open the bleed nipple and push the piston back all the way, then push some fluid through either using pressure bleeder or the conventional way pushing the clutch pedal in, close valve, release etc. once you have clear fluid bolt back to the car (you can use a small G clamp to hold the piston in while bleeding slave cylinder)
Mike Ellsmore

You have a faulty master cylinder. When the master piston as at rest against the end plate, there will be (should be) a clear path from the cylinder bore back into the reservoir. First verify that the pedal pushrod hs some minimal clearance, a small wiggle of the pushrod or the clevis pin. If not, then shorten the master pushrod until it does have a wee bit of clearance. If the master piston is not allowed to return fully to rest position, then fluid cannot return to the reservoir.

If the master piston is returning all the way to rest, then remove the top cover from the master cylinder, and remove most of the fluid from the reservoir. Find two hoes in the bottom of the reservoir over each piston. Look down into the hole over the clutch side, farthest from the end plate. You might expect to find this hole covered by part of the rubber cup, where the cup should draw back far enough to leave this hole open.

Solution for that problem could be to install two paper gaskets for the end plate, and cut holes in the gaskets large enough to clear the cylinder bore, so the piston can extend slightly from open end of the bore before hitting the end plate.
Barney Gaylord

This thread was discussed between 29/10/2021 and 30/10/2021

MG MGA index

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