M38 Master Cylinder

[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

[super dave]

My Big question to you is are you using Dot 3 or Dot 5 silicone brake fluid ?? The Dot 5 silicone fluid is in my words CRAP. I have had multiple problems in club members vehicles lately as the silicone fluid Swells the seals and does not return the piston most of the time or is very slow in returning at all, I have been using Dot 4 as its a bit of an upgrade to the Dot 3 fluid.

[David Herbert]

I agree with SuperDave's post but the reason that Dot 4 was developed was to make it more tolerant of the heat generated by modern disk brakes and to reduce the tendancy to absorb water from the atmosphere which would then boil because of said heat. As 70 year old brakes do not heat the fluid to any noticable extent the superior properties are rather meaningless. However it is easily available and will be totally compatible. Silicone fluid can be very hard to bleed as it can go around the bubbles rather than pushing them along the pipe to the bleed nipple.

David

[Bob Carriere]

On the a brand new system in my C15A I opted to use the Purple army surplus synthetic brake fluid years ago without any leaks, swelling, etc...... easy to bleed with a vacuum pump..... mine is due for it annual flushing..... suck the old stuff from the individual wheels until I get clear purple color coming out... then adjust each wheel, both bottom and mid cams, while the truck is on jack stand. A good opportunity to check each and every nut on springs, all u-joint bolts, etc. and verify fluid levels in axles, T case and tranny. Engine gets SHell T4 diesel oil and fresh dual PH 8 oil filters. Usually takes me a full Saturday.

Cheers

[Richard Whelan]

An update to those who may be interested in how this turned out. I am using Dot 3 for fluid. After not being able to get to my jeep for a couple of days. I decided to first use the test to crack the line at the master cylinder to see if the vacuum if indeed that was the problem would release the piston allowing it to travel normally. Well I did not need to go further. The piston is even slower today than it was when I first posted my problem.

So I removed it and benched tested it without fluid and the line ports open and the problem still occurred. So the problem is still in the master cylinder. I could try and bore it out a little more but I am afraid it will start leaking. After toiling with this beast for about a week I think it is time to move on and get me another.

Thanks for all the advice. Even though I could not resolve this it was indeed a learning experience I don't want to repeat.

Richard

[rob love]

Personally, when it comes to brakes and the time that can be spent trying to remedy a problem cylinder, I just put on brand new. Besides my personal collection, I also maintain a fleet here at the museum. I have not rebuilt a wheel cylinder since I came here....always brand new. It's cheaper than the labour and far,far more dependable.

The only time I actually even dis-assemble a master cylinder is when I am converting M35 master cylinders for use on M135s. A check valve has to be removed to make them work on that application.

In my experience, a rebuilt master gets you about 4 years, and a new one 7 years or better.

[Richard Whelan]

Not sure where to post info like this but for those looking for a Master cylinder at a good price. After spending some time trying to find a new Master Cylinder I found one through a company called Dorman and ordered it through Canadian Tire. Total price 55.00 Plus tax. It is the best deal around as far as I can see. You can order them on Amazon for as low as 63.00 CDN with free shipping. They sound like they specialize in this area and might have other hard to find Master cylinders for other vehicles.

Richard