M38 Master Cylinder

[mel gabel]

I rebuilt the master cylinder on my M38 and then proceeded to try and bleed the brakes. I encountered problems similar to what you describe and discovered that I had blockage in the flexible line between the frame and the rear diff. Replaced that line and the bleeding process went smoothly. The problem was not with the master cyl at all.

[Richard Whelan]

I cannot confirm any damage to the cups but they do look ok and it did bench test ok. I have been thinking it is a bad master cylinder but wanted to have some other opinions in case I was the problem. A friend of mine says he's never seen one like mine. It will probably be cheaper to purchase a new master cylinder but I will check my flex lines to make sure they are not the problem first.

Thanks Gents

[Bob Carriere]

making sure they are disconnected at both ends.............. not a bad idea to introduce some ethyl alcohol into the lines as a flushing agent...... large hypodermic needles are available cheap at Tractor supply....... then blow the line dry before reconnecting.

Not sure on the M38 Master cylinder.....but on CMP and other older GM models they have a tiny, needle size, bleedhole in the bottom of the MC...... you returning piston, after pumping, must clear that hole for the MC to work properly...... CMP MC habe a screwed adjustment on the push rod that fits into the MC..... playing with this adjustment will allow the piston to clear the pinhole in the floor of the MC.......PIN HOLE is visible from the filler opening.

Good luck.

[Richard Whelan]

I should've mentioned I installed new lines and flex lines last Fall. Since I made all the lines I will confirm no lines and flex lines are crushed or have some sort of debris inside just to be sure.

As well all my brakes pads and wheel cylinders are also new.

Don't want to purchase a new master cylinder just to find out one of the problems pointed out here was the problem.

Thanks

[David Herbert]

If the master cylinder does not return after pushing on the brake and is all connected up and blead as well as possible you could try pushing the brake and while it is trying to return (might take two people- one with a spanner one pressing the brake) crack open the hydraulic fitting on the outlet of the master cylinder. If it snaps back that would suggest a pipe blockage - usually a flex hose with no hole up the middle because the rubber has swelled. Enough pressure when pressing the pedal but not enough from the return springs. Also crushed metal pipes do happen.

David

[Bob Carriere]

Hi Richard.

On the outside chance that one of the newline or flex line is plugged..... I would still recommend a good blowing od all lines...... then you can focus on the MC..... Eastwood auto parts even sell a special can of ethly, pressurized like a paint can, to flush out new lines of any manufacturing left over oil..... or well hidden gift from a mud wasp!!!!!!

Good luck and keep us posted of your eventual solution.

[Richard Whelan]

All good points. Will do the work and post how it worked out. I should be able to fault find the problem with all the advice posted. More to follow.

[Lang]

Here is are some diagrams of all the bits of a master cylinder.

This is the operation of the bypass port (the little hole you refer to)

When brake pedal is pressed, push rod moves the piston against the force of the spring to the right,till it covers the bypass port. Further movement of push rod creates pressure in the compression chamber. When sufficient pressure builds up,the fluid is forced into the lines via a fluid check valve which is not marked here though.

When the pedal is released,the spring pressure moves the piston back,but there is a delay in the return of the fluid from the lines back to the compression chamber. This produces a vacuum in the compression chamber and unless this is destroyed immediately,there are chances of air leaking into the system. Since air is compressible that is not at all desirable. To avoid this,we use the intake port. As soon as some vacuum is formed fluid from the reservoir enters the compression chamber. But by the time vacuum is destroyed, the fluid in the lines come back into the reservoir.

This extra fluid has to be accommodated because the compression chamber is full. This is avoided by the bypass port. The extra fluid coming from the lines passes to the reservoir through the bypass port.

Lang