M35A2 Brake master cylinder does not return.

[Ratchetop]

When I use the brakes the pedal gets a lot of free travel between the master cylinder and the pedal plunger. The master cylinder is staying in. I am not very familiar with the system. I have been reading the TM'S. I installed new parts in the master cylinder and vacuum bled the brakes. The longer
you go the worst it gets.
Thanks for any help you can give.

 

[frank8003]

Is the MC vented and to where is it vented?

 

[Ratchetop]

I installed a remote brake fluid resiviour on the fire wall.

 

[frank8003]

I installed a remote brake fluid resiviour on the fire wall.

Did you remove the splash plate from the underside of the MC cap as suggested in the Ross write-up?
see attached beginning of page two or try this link
http://www.roscommonequipmentcenter.com/nn17.pdf

The vent tube port of the master cylinder fill plug is
threaded for 1/8-inch NPT. Below the vent port is a
splash plate that prevents fluid from splashing up into
the vent tube. This must be removed for the new fluid
reservoir system to operate properly. Use a punch to
knock it out.
• Install the 90° elbow into the 1/8-inch NPT port of the
fill plug.
• Reinstall the fill port plug into the master cylinder.
The vent tube port of the master cylinder fill plug is
threaded for 1/8-inch NPT.

View attachment roscommon MC nn17.pdf

 

[frank8003]

One may find this information useful

http://www.steelsoldiers.com/showthread.php?60159-Why-GL-Deuces-have-no-Brakes/page3

and this one

http://www.jatonkam35s.com/DeuceTech...ckheed1944.pdf

View attachment TM9-1827c Brakes 1944.pdf

 

[Ratchetop]

Did you remove the splash plate from the underside of the MC cap as suggested in the Ross write-up?
see attached beginning of page two or try this link
http://www.roscommonequipmentcenter.com/nn17.pdf

The vent tube port of the master cylinder fill plug is
threaded for 1/8-inch NPT. Below the vent port is a
splash plate that prevents fluid from splashing up into
the vent tube. This must be removed for the new fluid
reservoir system to operate properly. Use a punch to
knock it out.
• Install the 90° elbow into the 1/8-inch NPT port of the
fill plug.s
• Reinstall the fill port plug into the master cylinder.
The vent tube port of the master cylinder fill plug is
threaded for 1/8-inch NPT.

View attachment 652727

i checked it and the splash plate was still in there. I removed it and will bleed the brakes again to see if it makes a difference. I will let you know. Thank you for the info.

 

[brit4x4]

Hello, I am having the same problem the brake master cylinder plunger is not returning immediately when I release the brake pedal.
It was all working fine and then I did not use the truck for a couple of months. Checked the brakes and the pedal went loose, initially thought that there was a leak. Ended up replacing the master cylinder and air pack both NOS. Bled the system and the problem is still there.
I do have the remote reservoir, however I added a shutoff valve to the line so that I could disconnect the remote reservoir without losing the fluid.
Testing without the reservoir connected still produces the same result.
What else could be causing the slow or non return of the master cylinder plunger ?
Any help will be appreciated cannot move the truck without brakes.

 

[Jeepsinker]

First of all, make sure your pedal return spring is present. If that isn't it, consider overhauling the master cylinder ( maybe again) . If the vent is clear, thus can only happen if 1: the fluid ports in the reservoir are blocked with debris. 2: the piston return spring is weak, or 3: the piston packing has swelled due to age or incompatibility with whatever brake fluid you're using.

On that last note, don't use a new old stock overhaul kit unless it's dated in the 90s or later. Some of the older kits weren't meant to be used with DOT5 fluid.
I suggest using a new production kit from Big Mike's motorpool.

 

[brit4x4]

First of all, make sure your pedal return spring is present. If that isn't it, consider overhauling the master cylinder ( maybe again) . If the vent is clear, thus can only happen if 1: the fluid ports in the reservoir are blocked with debris. 2: the piston return spring is weak, or 3: the piston packing has swelled due to age or incompatibility with whatever brake fluid you're using.

On that last note, don't use a new old stock overhaul kit unless it's dated in the 90s or later. Some of the older kits weren't meant to be used with DOT5 fluid.
I suggest using a new production kit from Big Mike's motorpool.

Hello,
Thank you for you reply.
The pedal return spring is present and does return the pedal.
The master cylinder and air pack are NOS from Big Mike and have the same problem that the previous master cylinder and air pack had.
My 1967 M109A3 is older and has not had this problem and is driven way less than the 1975 M35A2.
I have rebuild kits from Big Mike that I can use. I am using Dot 5 from Big Mike.
But before I go through the PITA task of removing the master cylinder could there be any downstream issue that is causing this ? wheel cylinders, hoses ??
Best regards
John

 

[brit4x4]

Hello @Jeepsinker,
You were correct the problem is only in the Master Cylinder. TLDR; The little rubber valve at the end of the spring may have perished shut preventing flow in either direction. Interesting that the same problem showed up on the replacement, therefore age of master cylinders is probably the cause.

I had some time today to take the previous master cylinder, also NOS, apart. Tried various things such as clearing the return hole with compressed air but the piston was still slow to return.
Found my Master Cylinder rebuild kit, has AZ860513 on the box. The piston was a slightly different shape, I installed that but left the rest in place. The piston was returning but no fluid was coming out when I pushed the piston.
Took it apart and replaced the spring with the little rubber valve thing on the end. Now the piston returns and there is fluid expressed when I push the piston.
I am going to see if I can replace the parts in the master cylinder on the truck without having to remove the master cylinder.
I will provide a further update.

 

[Jeepsinker]

Ver

Hello @Jeepsinker,
You were correct the problem is only in the Master Cylinder. TLDR; The little rubber valve at the end of the spring may have perished shut preventing flow in either direction. Interesting that the same problem showed up on the replacement, therefore age of master cylinders is probably the cause.

I had some time today to take the previous master cylinder, also NOS, apart. Tried various things such as clearing the return hole with compressed air but the piston was still slow to return.
Found my Master Cylinder rebuild kit, has AZ860513 on the box. The piston was a slightly different shape, I installed that but left the rest in place. The piston was returning but no fluid was coming out when I pushed the piston.
Took it apart and replaced the spring with the little rubber valve thing on the end. Now the piston returns and there is fluid expressed when I push the piston.
I am going to see if I can replace the parts in the master cylinder on the truck without having to remove the master cylinder.
I will provide a further update.

Very nice. Glad you found the issue. Yeah unfortunately age and rubber parts aren't friends.