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MG Midget and Sprite Technical - necessary to bench prim clutch master cylinder?

I rebuilt my old clutch master cylinder and then installed a speed bleeder. The speed bleeder has a one way valve that only lets stuff out (no air can come back in so you don't have to keep closing it during bleeding).

Anyway, so I thought that I could start with my new cylinder, add fluid and then start pumping. However, I could not push any fluid out the bleeder (and there was not resistance). So I thought I messed up the rebuild.

So I installed a new clutch master cylinder. I tried the same process again and same thing.

Is it possible that an un-primed cylinder cannot pump any fluid no matter how many times you pump it? Since this happened with both cylinders I am wondering if I have nothing wrong with my master cylinder but instead I am priming it incorrectly.

Do I need to bench prim it? Could I force pressure into the top of the reservoir and get the fluid to come out (such as if I use EZ-bleed or something like that)?

My EZ-bleed kit does not have a cover that fits my new clutch master cylinder. I could get a spare cover and modify it. Would this work or do I need to do this on the bench?

Do I also need to do the same for my brake master cylinder (my brake system is currently completely dry - I have not tried adding fluid and bleeding it yet).

Thanks for your comments.
Rebecca
R Harvey

Rebecca -
Spridget clutch hydraulics are notorious for this. The fundamental problem is the large diameter steel pipe routed high above/behind the battery across the car and well above MC level. It holds about as much fluid as the cylinder displaces, so it tends to have a big air bubble in it that just moves back and forth as you try to bleed it.
The ideal fix is to run a new pipe from MC to slave, routed along the front edge of the heater box shelf. That makes the high point a short section at the back end of the MC, which bleeds OK. Failing that, you need to get that pipe full of fluid.

So, EZ Bleed might do it; I've used a piece of radiator hose long enough to reach well above the pipe and clamped to the MC filler. Pour it full of fluid and let it run out the bleed screw (might not work with a speedi bleed). People also unbolt the slave from the gearbox and let it hang down; once the slave fills up you close the bleed screw and push the slave piston back, which quickly forces fluid and air <we hope> back through the pipe into the MC. Paul Hunt suggests connecting a hose from the RHF brake bleed nipple to the slave nipple, and using the brake system to backfill the clutch hydraulics.

You don't usually have as much trouble with the brake side, though it uses the same stupid pipe routing, since the pipe is smaller diameter.

Remember in all this that brake fluid removes paint! Brake fluid is water soluble though, so washing stuff off with water quickly will save it.

FRM
Fletcher R Millmore

The trick I've used previously is to connect the easi-bleed to the slave cylinder nipple and then crack the union at the master cylinder - have pleanty of rags around to make sure no fluid gets onto the bodywork. The air is expelled and fluid will eventually weep out of the union. Tighten the union and then bleed as normal from the master cylinder end if necesssary (I don't bother with this final bleed as the clutch works fine without it)
Andrew Dunn

I wonder if you couldnt just jack the side of the car up on the USA drivers side....Use a 2x4 wooden stud about 3-4 feet long going from front to rear wheel under the sill and a floor jack to raise the hole side up both drivers side tires off the ground...a good 12-18 inches or so

that would put the MC higher then all the lines and slave clyinder...and allow the air bubble that FRM is refering to float to the top side of the MC.


Not sure if that would work...but I dont see why not, granted you will have to work the bleed valve while sitting on top of your head in the passanger side footwell, hope your limber...LOL

Prop
Prop

This thread was discussed on 12/04/2010

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