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Hi folks,

This is really interesting.
I jumped in my rig, turned around to hookup my plow and discovered that I had zero resistance with the brake pedal.
Brakes have been working perfectly for years under very low use.

There are no visual leaks and nothing obvious when depressing the pedal.

The only work that I have been acively engaged with is the removal of the Hydraulic Pump and Motor for the Plow.
I inspected the remaining parts and they were not jerry-rigged to work with the brake system
( You never know )

Is this a normal failure ?
Is there a diaphram that has let go in the MC ?

My next step is to dig through the TM's and understand the syste, end-to-end.

I just find it odd that I have a total failure with no obvious signs ?

What is the fluid level? It is a single circuit system. Low fluid, busted line, failed wheel cylinder, failed master cylinder will all give that symptom.

Hey there
It's the original MC.
I'll take a look at the fluid level this weekend, I was so supprised that there wasn't any leaks, given it stopped working all at once.
However in the Manual, all reasons for "Loss of Pressure" lead to some form of fluid loss somewhere in the braking system.

Here we go !
The good news is I have a rolling chassis with lots of extra parts.

about a month ago my pedal went to the floor. pulled all four drums no leaks. pulled apart the master cylinder no obvious defects. bled the system and brakes felt good. went to the truck today and the pedal went to the floor. I suspect it could be the residual pressure valve inside the mc because the pedal gets hard with a few pumps then gets soft after a few hours of sitting ?

most likely a bad cup (piston) in the master cylinder.

I had the same problem, I would have great pedal after adjusting and bleeding the brakes, then lose pedal after sitting for awhile. I have a dual master cylinder and replaced it once, still had the problem. Finally I replaced all the wheel cylinders, even though I didn't see fluid leaking, and haven't had loss of pedal since. My theory is that it was getting air past the wheel cylinder cups while sitting, the old wheel cylinders were pretty rusty inside. When I put the new wheel cylinders in I took the pistons out and coated them with silicon brake lube to keep them from corroding and sticking. Its been working great for a couple years now.

Joe

Mystery solved !
The rear brake line was perforated and leaking at a rusty spot.
I guess it got to a point where the wall was so thin, the smallest amount of pressure ruptured it.

Very interesting.

I compared the newer MC on my truck with the older parts doner.
The Parts Donor still had the switch with Douglas connector on it, very interesting.

Does anyone understand the purpose of the Master Cylinder to Air Cleaner Tube ?
It's mighty hard to track down in the TMs.

The tube to the intake elbow is the vent for the MC. There is no hole in the plug or lid for the MC for the deep water fording system. Keeps dirt and water out of the brake system. ( There should be a valve to isolate it at the MC. )

just a thought. If one line failed, then the others may soon do so at a most inopportune time. I re-did all my hard lines with copper-nickel lines from FedHill, it was easier with the bed and engine out, but we can share some hints about how to do that, and where to source parts, and suggestions as to a good flare tool source if you wish. A search on this forum will also yield past experiences with replacing lines.

NV

Yes Please !

I will indeed replace all hard lines.
Funny, the Flex Lines are all in great condition, must have been changed at one point.

I looked at MidWest's Hardline Kit and through the M37 Alternative Parts Finder to see what options I have.

If you guys have a source that has good product with decent prices I am totally interested.


Thanks so much for the info about the vent, I figured it was likely for Fording if it went up to the Air Cleaner dish.
The MC on my truck had it completely capped off, along with the b/u switch location, as it was used as a Gas Station Lot-Plow.

I'll hunt for replacement threads to see what I'm up against.

You guys are THE BEST !

I did the kit from VPW. After looking at what was involved, I did the fuel and vents at the same time. Laying under the complete truck it was a two evening filthy dirty job.
Wasn't hard, just dirty.
Fixed a lot of issues with fueling and brakes.
BTW, I have the tools and ability to form and flare new ones from scratch. Buying preform took a week out of the job. Time is money.

Thanks so much for the pointers.

I'm guessing you picked up the Steel Lines from VPW ?
Was there anything else when you were under the Truck, that made you say, "If only I'd ordered that xxxx, life would have been so much easier " ?

Best regards,

I needed some customized line wrenches. Bit of heat and that was accomplished. Copper washers for the fittings are nice to have.

Dang Nab It! I was reading and following this post, went out to my son's M35A1 to crank it and let it run. The brake pedal goes to the floor.

that sucks! sympathetic brake loss. the big guy must have felt for its little cousins.

I was blow away today in my hunt for a fix as my local Lordco had a raybestos mc kit for $19ca ($14usd)