VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc Trouble bleeding brakes

Howdy folks, amateur mechanic here working on a car that is almost thrice my age and in need of your opinion!

Last spring I pulled out my master cylinder because pressure was building in the lines and locking up the brakes after about 12-15 cycles. I broke the casting trying to get the brass fitting out the back, but it turns out the inside was melted anyway, so I found the right one on ebay and ordered it. I cleaned it up (cleared the tiny clogged relief port, to boot!) and put it back in last fall. Fast forward through a busy and dreary winter to this morning with me trying to bleed the brake system, but failing.

Obviously, I don't have the autofiller do-hickey that the shop manual references, but surely this can be done without that specialty tool. I filled the MC about 3/4 (manual said 1/2, but I am an over-achiever) and starting with the back left, attempted to pump the brakes and push fluid through the lines. Unfortunately, it wants to shoot fluid out the top rather than the port at the back. If I cap the filler hole, it squirts fluid out what I'm guessing are pressure relief holes in the sides of the plug at about 3/4 of the full pedal stroke. I disconnected the main brake line from the back of the MC and nothing comes out the back when pumping the pedal, so does that mean something inside the MC is clogged still? I don't suppose finding a plug without those little relief holes would allow me to get more pressure and force fluid through?

Car is a 1939 Chevy sedan with a floor shifted 3spd. Previous owner put on a front disc conversion, also. Thanks!

I'm not aware of what relief holes are in the sides of the plug. -- So I'm repeating that here so that someone else can jump on it.

And, if there's nothing coming out the back (main line), then that would certainly indicate the problem is in the Master Cylinder.

If you can't diagnose a fix for it, then it sounds like you need NUMBER THREE. Post a WANTED AD in the Classified forum .

Perhaps you enlarged the clogged return hole when you cleaned it. If too big, the brake fluid will shoot up to the top of the master cylinder and come out through the cap vents. I learned the hard way that this is critical when I used a 1/16 drill to open a clogged hole in one. If this is the problem, you might try to fill the return hole with liquid metal and then drill out with the smallest bit you can find. The return hole should probably be about 1/64 diameter.

Given the description of clogging and master cylinder condition, I wonder about the brake lines and fittings condition. If you decide to work on those, I learned the hard way, make sure to use the correct brake line wrenches and be especially careful with the brass junction blocks.

Front disc brake conversion...Are you still running the original Master cylinder...?

I used a needle and pb Blaster to clear the return port, so it shouldn't be enlarged.


I'm wondering about them too.... I think I'm going to order a new set of hard lines and inspect the junction blocks. The lines aren't that expensive from Filling Station and after all, ya gotta have brakes, right??


Yep. Well, original style at least, I hulked my original one into pieces . From what I understand, the discs should work fine without further modification to the brake piping. I have heard mention of a proportioning valve, but I reckon it's just something I'll address once the brakes are all put back together.

Thanks for all the responses, ya'll! Looks like I'll pull the MC back out and make sure it's functioning right and then go ahead and replace all the hard lines.

PanHead

I have a '40 and over the years I've had the MC spurt fluid back up thru the fill hole several times. Each time it was because the primary cup in the MC was bypassing (leaking) fluid. The primary cup was was either worn out or installed backwards (Yes, I make mistakes occasionally). So check the internals of the master cylinder and make sure they are all OK. I have had the car for over 60 years and have never had a brake line clog up or block.

I am not sure about PB blasterbut WD40, oilo and similar will react with the neoprene/rubber brake components causing them to swell. Brake fluid is a vegetable oil base.
If this is the case pull the m/cyl and replace the rubbers.
Tony

If he's contaminated the brake system with Blaster, he's probably going to ruin not only the rubber parts in the master cylinder, but, the rubber parts in the front calipers and the rear wheel cylinders as well.

Regards:
Oldengineer

Yes. I'd think of replacing the rubber...Even brake fluid will swell poor quality rubber...But, I can't seeing it falling apart and clogging the line...

In all the m/cyl I have seen the primary cup (the 1 nearest the spring) in normal released position only just uncovers the compensating port (smallest port in base of reserviour) so any swelling soon closes this port causing several problems.
Tony