1940 Plymouth brakes

[Go Fleiter]

So, Ken, no more obstakles for Your Saturday in Indy! Wish I could be there! Have a nice Day! Greetings! Go

[Ken Bartz]

The problem turned out to be the check valve as it was not functining as it was suposed to.  I installed a new master cylinder kit which contained a new check valve.  Then after beeding the brakes and doing the minor adjustment everything worked as expected.  Thank you everyone for your help.

[brucepine]

The valve assembly (one way check valve) does need the O-ring installed first in order to seal the check valve at the cyllinder end.  The spring will hold the check valve tight to the cyllinder end and the (one way) action works.  Without the O-ring the valve is metal to metal and will not seal properly.   Hope this helps.

Bruce Pine

[TodFitch]

I don't see a rubber washer in the diagram in the service manual either.

The push rod adjustment is the item (1) in the quote I posted earlier: The rod should not be touching the piston when the pedal is released. The pedal should travel between 1/4 to 3/8 of an inch before the rod touches the piston. You should be able to feel it when you press the pedal by hand.

[Ken Bartz]

Thank you for your reply Todd.  How long should I have the pedal push rod?  I have 16 threads showing on my current setting of the push rod.  But, looking in the parts manual that has about 10 threads showing.  Probably best to have a shorter length as I can adjust longer later if needed.

And what about the rubber washer that comes with the rebuild kit that gets placed in the master cylinder first, before the valve assembly.  The parts diagram does not have this item showing.  I'm tempted not to install it.  It is installed in the new replacement master cylinder.

[TodFitch]

I concur with you that it sounds like the master cylinder.

One thing that kept getting me was the pedal travel, it turns out that you cannot get the pedal up where you expect it in a modern car. Pedal travel, the distance the pedal travels before it becomes firm, is going to be between 1-7/8 and 2-3/8 inches. Quoting from the 1936-42 service manual (distances at the pedal):

(1) Free play ("A," fig. 1) in the brake pedal should be from 1/4 to 3/8 inch. This free play may be readily felt by hand and is the movement of the pedal before the push rod touches the master cylinder piston. If necessary, this free play may be altered by changing the length of the piston push rod.

(2) After the free play is taken up, and the pedal is pushed slowly for an additional 5/8 to 3/4 inch ("B," fig. 1) fluid should be forced up through the relief port in the master cylinder. If it does not, and the free play is correct, the master cylinder should be disassembled and checked for swollen cups or improper assembly of parts. The additional pedal travel of approximately 1 inch is required to move the brake shoes outward against the brake drums.

So you have up to 3/8 inch before the rod contacts the piston. Add 3/4 inch for piston travel before it covers the relief port. Then add an inch for moving the brake shoes up against the drum. 3/8 + 3/4 + 1 = 1-11/8 = 2-3/8 inches.

If you determine it is not an adjustment problem. . . There is a valve at the outlet of the master cylinder. It should allow fluid to move freely into the brake lines but should restrict flow back into the master cylinder. This does two things: 1) It allows you to pump up the brakes. 2) I keeps some residual pressure in the system which helps keep the wheel cylinder cups sealed against their cylinder bores. I suppose it is possible to assemble the valve assembly in such a way that no return to the? master cylinder is possible. That would cause the same symptoms as a blocked relief valve port.

[Ken Bartz]

Here we go again, trying to solve the brake problems on my 1940 Plymouth.  When I drive the car the brake pedal get harder and harder, eventually locking up the brakes and the brake light comes on solid.  The classic symptom of the master cylinder relief port being plugged.  I can take the cover off of the master cylinder and insert a wire in the relief port.  It does not appear to be plugged that way.  I have read that it might be possible that the master cylinder piston might be too long, thus covering the relief port.  It is a new one that came with the complete new master cylinder.  I found my old master cylinder piston and it appears to be identical.  I thought that it might be a flex hose breaking down causing a one-way valve.  But that would give the symptom of just one (or the other) front wheel or both rear wheels locking up.  With the brake light on the pressure has to be in the whole system as the brake switch is right at the master cylinder.  I have now rebuilt all wheel cylinders with new rubber and adjusted the shoes with the Amico 1750 tool to .006.  The master cylinder is removed and dissembled waiting for a solution to this problem.  When the system is back together it will be bleed with the pressure bleeding tool. 

Questions:  What should the length of the piston push rod be? 
My rebuild kit comes with a rubber spacer/washer that would fit behind the valve assembly. Should this be installed?

Does anyone have an idea what would cause this problem?  It is real pain to remove and install the master cylinder so I would like to get it right this time!