How difficult is replacing the entire braking system on a 1971 M35A2?

[mchagala]

My 1971 M35A2 has been parked, without running, for several months. I got it running today, only to discover the brakes no longer work – the pedal goes to the floor without resistance. While looking for air leaks and other issues, I realized most of the components, like master cylinder and air pack, are rusty and corroded. I am considering replacing the entire braking system (keeping it stock, with refurbished/NOS replacement parts). Safety is my priority, not cost.

I’m not a mechanic but am pretty handy and willing to buy the necessary tools. The master cylinder and air pack look straightforward to install and replacing the lines seems like mostly a matter of buying the right tubing and getting the bends and connections correct.

Here are my 3 questions:

• Am I about to get in over my head and not be able to finish this?
• Which is safer: fixing the old rusty system, or a new brake system installed by a beginner?
• Which is safer: refurbished vs NOS air pack (shorty). Cost is not a factor, only safety. Refurbished will have some amount of wear and tear, but NOS has seals and gaskets that might have been sitting on a shelf for decades.

Any input would be helpful, thanks!

 

[NDT]

Does your brake system have silicone fluid (BFS) in it from the military? If so just top off the master cylinder and rebleed. If someone put in nasty regular brake fluid, yup you are looking at an end to end overhaul and line flush. Many hours of work.

 

[mchagala]

Ok, my thinking on this has evolved a bit. I am really asking 2 separate questions in this thread:

Why did the brakes stop working?

While it seems odd to me that brakes could go from great to non-existent without any visual sign of a leak, this can simply be proven by topping-off the fluid and seeing if the brakes start working again. If yes, then the easy answer is to keep driving and check the fluid level often.

When to replace the entire system?

At what point does it become safer to replace the entire braking system vs repairing the old? Again, cost (and time) is not an issue. The brake system doesn’t appear to have been maintained in any way since the 80’s, but at the same time, the brakes were working great up until recently.

 

[NDT]

For reasons I don’t understand, silicone brake fluid magically disappears from deuce master cylinders with no signs of leakage.
Sure you can overhaul the system using now available crap repro parts. May not gain much reliability.

 

[swbradley1]

I would follow NDT's advice first. Build or buy a power bleeder. Build an adapter to connect it to the top of the master cylinder. Fill it and start bleeding. Do the first bleeder on (I believe) the air pack. Then go to the farthest away (right rear) and then 2nd farthest etc etc.

Then adjust the brake pads.

If it all works you are money and time ahead.

 

[swbradley1]

Amazon.com

www.amazon.com

The Motive bleeder. It lets you bleed the truck all by yourself.

 

[mchagala]

This is good info, thanks!

 

[mchagala]

I found information that backs up what you both say. This video says having to bleed the air pack is normal and expected when the truck has been sitting for a while. This is good news not only in that it’s an easy fix, but also that this is not indication of a larger problem. I think I’ll save the system overhaul for another day…

M35, M35A2 Brake Pedal to the Floor! Bleeding the Airpack First.
Tim's Teachable Moments

 

[cattlerepairman]

Yes, I can attest to the "disappearing" silicone brake fluid. I thought, for sure, it had to be leaking into the air pack. Took it out to overhaul..not a drop of brake fluid. No visible brake cylinder leaks. I have resigned to topping up brake fluid after winter storage and not ask silly questions. Brakes work fine and have for years.

 

[mchagala]

The truck is up and running again. I was able to get in touch with the previous owner and confirm Dot 4. I used a Motive Bleeder to flush out the old fluid and fill with new. Brakes are now firm. Thanks for all the help – I’m new but learning fast.

 

[NDT]

Make sure you do flushes every few years. And check the master cylinder level before road trips.

 

[HDN]

Yes, I can attest to the "disappearing" silicone brake fluid. I thought, for sure, it had to be leaking into the air pack. Took it out to overhaul..not a drop of brake fluid. No visible brake cylinder leaks. I have resigned to topping up brake fluid after winter storage and not ask silly questions. Brakes work fine and have for years.

This is one of the top questions next to the meaning of life: Where does my brake fluid go?

The only answer I can think of is that more fluid sits in the lines as a result of brake shoe wear and the wheel cylinders not fully retracting after the brake pedal is released. But that wouldn't be normal brake system behavior because the cylinders should fully retract after the brake pedal is released, right?

 

[Barrman]

I made the linked video and then installed a see through remote reservoir. Here is the video about that. Here is that video:

9 months later I have to say the remote reservoir is something I should have done years ago.

 

[mchagala]

Crazy. I found a random video on YouTube that described my problem and posted it earlier in this thread. Then a few days later the actual guy from the video replies! God bless you and everyone else who makes videos and helps the rest of us keep our trucks running!

 

[swbradley1]

One of the best first mods I made to my first Deuce was a remote reservoir. Every time I opened the door it was right there behind the seat in my case.

 

[Barrman]

I’m glad the videos helped. When I started doing the video thing I made a list for each of my vehicles of things I had issues with. That is what I do the video’s about. Especially if I have had to do the same repair multiple times.