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  #1  
Old 18-06-18, 17:17
Richard Whelan Richard Whelan is offline
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Default M38 Master Cylinder

Anyone out there know how to diagnose a master cylinder problem on my M38.
It is new. The piston would not return properly when I first bench tested it on my vice. It was well lubricated but would not work properly. So I honed out the bore until the piston moved freely. Bench test it again with brake fluid and no leaks were found so I installed it and started to bleed my brakes.

Now here is where the problem begins. When I bleed a wheel cylinder the piston takes about 2 minutes to return fully to the rear. I think there is some sort of vacuum problem but do not where to look. I think it is still in the master cylinder but not sure what to do about it. I've checked the intake and bypass holes and they are clear.

Note. When the piston finally returns fully the master cylinder functions correctly but the brakes are still spongy. So with the problem when bleeding a wheel cylinder this prevents me from removing all the air.

Completely confused
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  #2  
Old 18-06-18, 19:29
rob love rob love is offline
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Sounds like you got a bad one right out of the box. Any chance you damaged the cups by over-travelling the piston into the master cylinder?

Make sure you have free play on the rod to the master cylinder. Mind you that will normally lock up the brakes rather than cause slow return.

Personally, I use a bleeder ball to bleed brakes as opposed to using the master cylinder to create pressure. Alternatively, you can also use a vacuum pump at each bleeder screw....the pumps are available relatively inexpensively.

Master cylinders are readily available and cheap. I see on ebay where they are listed at $35USD.....worst case is NAPA where they are $100 CAD.
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  #3  
Old 18-06-18, 19:36
mel gabel mel gabel is offline
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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.
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  #4  
Old 18-06-18, 20:57
Richard Whelan Richard Whelan is offline
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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
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  #5  
Old 18-06-18, 21:52
Bob Carriere Bob Carriere is offline
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Default Try blowing the line......

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.
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  #6  
Old 18-06-18, 22:30
Richard Whelan Richard Whelan is offline
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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
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  #7  
Old 18-06-18, 22:37
David Herbert David Herbert is offline
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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
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  #8  
Old 18-06-18, 22:59
Bob Carriere Bob Carriere is offline
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Default Good point .....

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.
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  #9  
Old 19-06-18, 02:04
Richard Whelan Richard Whelan is offline
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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.
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  #10  
Old 19-06-18, 02:30
Lang Lang is offline
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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
Attached Thumbnails
Master Cylinder5.jpg   Master1.png  
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Last edited by Lang; 19-06-18 at 05:41.
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  #11  
Old 20-06-18, 06:52
super dave super dave is offline
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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.
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  #12  
Old 20-06-18, 11:22
David Herbert David Herbert is offline
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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
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  #13  
Old 20-06-18, 22:53
Bob Carriere Bob Carriere is offline
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Default Been using military grade for years...

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
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  #14  
Old 22-06-18, 15:23
Richard Whelan Richard Whelan is offline
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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
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  #15  
Old 22-06-18, 16:59
rob love rob love is offline
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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.
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  #16  
Old 23-06-18, 16:18
Richard Whelan Richard Whelan is offline
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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
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  #17  
Old 24-06-18, 09:14
rob love rob love is offline
carrier mech
 
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I suspect a lot of these cylinders are made in China, and the various companies put them in their own boxes under their brand names. Dorman is one of the better priced companies I have found. I use them for the Ford CMP wheel and master cylinders.
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