M39 master cylinder reservoir rusted

[Robo McDuff]

Took off the MC of my M51A2.



Despite the white color, the tag on it states its for a 5-ton only, and seems to be military origin.

During the several years standing without driving and topping it up a few times, I lost all the fluid in the MC reservoir, but some was left in the cylinder itself. As result, the inside of the reservoir looks rather rusty. The "cylinder" part looks ok, also the seals. However, there was some debris in it from the reservoir.

Anybody ever cleaned this? Maybe sandblasting the reservoir and then make sure it never gets empty again? Or clean (electrolysis) and paint with DOT 5 resistant rustoleum? Or some other trick that does not interact with the DOT5 later?

Yes, replacing the MC would be better, BUT ... I am living in the Czech Republic. So getting an MC from the USA here (and the other parts I need) costs a lot of $$$ in shipping costs. Up to 20 pounds already costs around 100$ by USPS. Trow in an air pack, some steering equipment etc, and you end up with shipping costs up or over $$ 1K!

The other thing is that I need a quick solution now (next few days, max a week) to have the truck movable on our private terrain and in the forests, and then a long term final solution. If the short and long term are the same, great. If not, a quick solution has to do for the moment.

What are possible civil replacements that might work which I can get here in Europe?

 

[red]

Wouldn't put paint inside the reservoir, eventually will flake off and plug things up.

The 5 tons don't have the limited space problem around the master cylinder like the m35's do. So a different master cylinder for another truck is easier to find. Check with some of the local heavy truck manufacturers (In the US companies like Freightliner, Kenworth, etc)

 

[M543A2]

I have taken them apart and cleaned them. If the cups are still good I put them back in, especially for a short term like you have. Many times they are OK and do not leak. If the rust you see inside is not flaky rust, just light surface, do not worry about it. If worse, carefully sand blast it after taping or otherwise plugging off the two holes in the bottom so the sand does not go down into the bore, possibly damaging the finish. Be sure to very thoroughly wash and blow out any left over sand out. With the internal parts out check the bore for damage. I have honed the bore a bit to clean it up and re-assembled them. Note that there are two holes in the bottom of the reservoir, one larger one and one quite small. Be sure both are clear of rust or debris. If the small one is plugged your brakes will not want to release properly. Also, if there is a heavy truck parts place nearby, take the MC and internal parts to them and see if they can help either with another cylinder similar or with rebuild parts if you need them. Making sure it stays full is the best way to prevent it from happening again.

 

[Robo McDuff]

I think I was too hasty with my judgement. Today I had the air pack and the MC on the bench, cleaned them, and inspected them again. With the MC, most of what I though was rust turned out to be debris, and after cleaning and inspecting with a strong flash light, the reservoir looked ok.



The cylinder looked a bit more rusty then thought on first view.



However, today I received a Kennedy honing set, so I will hone the cylinder, and put everything back together. I don't think new seals are necessary, they look ok.

As you see in the second picture, I have the two holes when looking in the reservoir. However, the smallest one looks solid, and when I check the inside of the cylinder, it looks like there is only one hole????? I will check again with a fine needle tomorrow, but the cylinder looked and felt smooth there. What is the function of he second hole so close to the first one?

 

[Robo McDuff]

MC cleaned, painted and rebuilt

I removed outside rust, then had the MC overnight in a bucket with water and baking soda using an old mobile phone charger to do some electrolysis and clean all rust inside.




After that, I softly honed the inside of the cylinder to make everything smooth again (using DOT 5 braking oil during honing). The inside of the reservoir I cleaned and got rid of all free rust or debris, and put DOT 5 brake oil as protective layer everywhere. Then I painted the outside of the MC with "blacksmith black" rough paint. Looks good now.

The famous second hole: had to grind down a small paper-wire to a very thin point, and found that it indeed extended from the reservoir into the cylinder. However, I had to push it through and dislodge debris even after cleaning. I would imagine that it will clog up again real soon.




At the end of the cylinder, behind the piston there should be installed "a plate and spring wire" to keep the piston from going out of the cylinder. You can see the spring wire in the lower right corner.

1) Cannot find the original plate, and a new plate was not in the rebuild set. Any odd thin washer the right size will do?

2) at the end of the cylinder, inside, there are two groves. The wire spring should go in the inner grove or in the outer grove or it does not matter, and why two groves?