Flushing the brake system on an M-39

Replaced and flushed through my M-39 (in my case M51A2) brake system today. I had to replace some brake lines and thought it was proper time to do so. What I did:

All but the right front line and hose were removed. Basically, each brake cylinder was disconnected from the main brake line. The lines from the Master Cylinder until the splitting blocks on the front axle and on the tandem stayed connected.

I opened the bleeding nipples and let the brake fluid run out of the cylinders (catching it off course)

Using a small compressor, blew air into the bleeding nipple blowing all old stuff out. I did not put the air gun directly onto the nipple but about 1/3 inch away, not knowing if the high air flow might pressure some seals out the wrong way. Maybe over-cautious, but it did work OK anyway.

Then I started on the Master Cylinder. The construction of the vent line had me baffled enough to go back to the computer and the manuals to check what the @#$%$ was happening there and which .... idiot has design this system. In the end, I just removed the vent line, unscrewed the cap, and start pumping the brake, wait a bit, pump again etc etc.

Cleaned the reservoir a bit with a dry cloth and filled her up with the nice blue stuff; fresh new DOT 5. What actually came out was yellowish, and the stuff coming out of the cylinders was a bit mixed and dirty yellowish as well. Anyway, kept filling and pumping with a completely disconnected system until the ends on the front axle and the tandem started spitting out clean blue stuff as well. With all leaking, took about 1 1/4 liter to get that result!

The front axle was spitting blue first, so I connected the new brake line and by the time the rear line gave blue, the stuff now coming out of the front brake bleeding nipples was blue as well.

Tomorrow will connect the new rear lines and keep pumping until those nipples spit blue as well. Then we will do a normal bleeding procedure, then start the engine, try some power braking, stop and bleed again.

Forgetting anything?

The yellowish stuff. Was that the old color of DOT 5, or was the whole system full of DOT 4, or the color coming out does not say a thing about what was in it?

Yup, dot 5 turns from blue/purple to a yellowish color over time.

gimpyrobb said:

Yup, dot 5 turns from blue/purple to a yellowish color over time.

Click to expand...

phew, that had me worried, thanks mate

I had just seen that in my Deuce.. yellow... and when I added DOT5 it mixed well.

I am glad I know its the same now.

Flushing completed, everything blows out blue, but by then it was getting late, and we did not manage to complete everything in time for tomorrow. #@#$%

But the truck looks nice with a fith wheel on it.

Got the flushing finished Sunday afternoon, all coming out nice and blue from all cylinders, and man, without engine, that peddle sticks high and has a lot of pack behind it not.

Great!