Brake Master Cyl/Lines advice needed

longld · Post by **longld** » Sat Apr 15, 2017 12:59 pm

Rebuilding my 53 M38A1 and brakes go to floor - Master Cylinder was empty of fluid - filled it and tried bleeding to one front and one rear wheel - no fluid or air. Pedal goes to floor easily and linkages look good. Suspect Master Cyl needs rebuilt and will do this next - when I remove this what is proper procedure to insure lines clear? Loosen all bleeder fittings and blow pressurized air or....? thanks!!!!

Post by **wesk** » Sat Apr 15, 2017 7:07 pm

Ex-military brake system usually left the military with DOT-5 silicone based brake fluid. The standard civilian fluids DOT-3 & DOT-4 are glycol based. The two are not compatible. The DOT-5 is the hardest to flush and remove all traces of. I won't get into the rediculous arguments between using DOT-5 and DOT-3 or -4. You can educate your self on what is the best choice to use from now on by just googling DOT-5 vs DOT-3/4. Suffice it to say I do not use DOT-5 and 99% of the automotive industry does not use DOT-5.

If you have no idea what your system has in it then just buy a can of DOT-3 and compare smells!

When dealing with an empty system you will normally have to fill the master cylinder several times. Now if you filled the master cylinder and pumped the brake pedal full stroke over twenty times and the master cylinder is still full then you have an issue with the master cylinder bypassing internally

In your case I would start by flushing the entire system with Iso-propyl-Alcohol. Strip clean with (Iso-propyl-Alcohol) & rebuild the master cylinder and all four wheel cylinders.

http://www.ucx.com/documents/brakefluidrev.pdf

skyjeep50 · Post by **skyjeep50** » Sun Apr 16, 2017 8:54 am

If your brake master cylinder was dry you may want to carefully inspect all the lines and wheel cylinders for rust and gunk. If the vehicle had not been properly maintained the brake system may be gunked up to the point that a alcohol rinse will not really do much good. When I started working on my M38 the brakes worked but a look at what came out of the master cylinder and lines convinced me I would have to re-accommodate some parts.
Replacing the lines/hoses is pretty easy and will assure you that you can use any brake fluid you choose without contamination from what was used previously.

whydahdvr · Post by **whydahdvr** » Tue May 09, 2017 1:02 pm

Wes,
I'm looking up brake fluid information. It looks like:
A) I need to stick with DOT 5 fluid for my 1952 M38
Or
B) It doesn't matter what I use. ..

Your earlier post about not mixing the fluid types makes sense. I need to top off the fluids so which do I go with? I think it's the DOT 5 but want to confirm first.

Thanks!

Post by **wesk** » Tue May 09, 2017 10:18 pm

The only way to confirm which fluid is in there is to smell it.

If you don't know what Dot 5 and Dot 3/4 smell like then buy a can of each sniff them and then sniff what's in your jeep.

B) It doesn't matter what I use. ..

This is not an option. You can't mix them period.

whydahdvr · Post by **whydahdvr** » Wed May 10, 2017 4:22 am

Thanks Wes.
We got it figured out. All set!

Furley11 · Post by **Furley11** » Wed May 10, 2017 8:22 am

whydahdvr wrote:Wes,
I'm looking up brake fluid information. It looks like:
A) I need to stick with DOT 5 fluid for my 1952 M38
Or
B) It doesn't matter what I use. ..

Your earlier post about not mixing the fluid types makes sense. I need to top off the fluids so which do I go with? I think it's the DOT 5 but want to confirm first.

Thanks!

I just did a full MC and line replacement in my M38. The cost and ease of doing it versus the problems that can arise (brakes going out) make it a no brainer to do a full replacement of MC and lines. Then, you can put whatever fluid you want to in there.