48 Special brakes

[ozzmonaut]

So, this started with slightly weak braking, but no sponginess. Then I determined that the front driver top wheel cylinder was leaking. The pressure was still good on the brakes, and was still able to stop fine. I changed all wheel cylinders, using the original pins from cylinder to shoe. I have the copper washers in place on the front hoses. I did have to loosen the shoes up front with the adjusters, but I marked and set everything back to where it was, since the brakes worked well before at these settings. Now I have bled the system multiple times, and have gone through about a quart of fluid. Still can't get any pedal. Given that the master was providing plenty of pressure when this occurred, that the shoes worked well where they were set, and that there are no leaks currently from any cylinders, lines, hoses, fittings, etc, what would be other causes of not getting pressure at the pedal? Thanks

Edited July 20, 2020 by ozzmonaut

[desoto1939]

The brakes either got air in the lines and you should re-bleed all for wheels starting with the right rear, then left rear, right front then left front.  Make sure you have no air bubbles.  The do a minor brake adjustment with the top cams?  Di you touch the eccentric pins when you did the brake repair? If you moved these than the shoes are not adjusted properly.

 

Let us know what you do so we can guide you through the process.

 

Rihc Hartung

[ozzmonaut]

Like I said, I had run about a quart through the lines bleeding. Now that's up to about a quart and a half. As a precaution, I had the concentrics marked. I now have the shoes tight against the drum on all wheels to aid in the bleeding process. Have been bleeding in order, furthest from the MC first.  I did not move the major adjusters. I marked where the arrows pointed to on the backing plate, and installed them the same.  At least when the brakes leaked, I HAD brakes. Now I have to ride a bike back and forth to work until I get this right.  I have a new MC on the way. The one on it was working fine, but I have no other ideas as to what to do. It should be a relatively simple fix. With the shoes against the drums, fluid has no option but to force out of the bleeders. There is no place for air bubbles to be hiding. The fluid coming from the bleeders seems to have no bubbles in it. It just comes out smooth and steady.

[plymouthcranbrook]

It does sound like an internal master cylinder problem to me.  if you gently push the pedal with the cap off does the cup move and the vent return?

[ozzmonaut]

I can see the cup moving, but difficult to tell if the vent is returning. I doubt the MC is the culprit, or else I would have seen issues before the wheel cylinder started leaking. But at this point, there are no more components to check. I am getting fluid from every bleeder, so there are no clogs or collapsed hoses. No leaks, so the fluid has nowhere else to go but into the cylinders. The shoes are against the drum walls, so there can be no excess movement of the cylinders to push the shoes. What else is left?

[9 foot box]

   With the shoes adjusted tight, the brake shoe springs can’t return the pistons to a brakes off position, returning fluid pressure back to the master cyl. Maybe read the brake section in the above Technical tab. 

[Dodgeb4ya]

Sometimes when pedal

Bleeding brakes on an old master cylinder you can damage/tear the master cylinder cups.

This is because you push the pedal beyond it's normal range. The cups get pushed into the ridge of possible sludge.

That can damage the rubber cups.

Or the piston can hang up on the return stroke...so as mentioned watch it's movement.

A reason to use a quality pressure bleeder.

And yes the shoes should be backed off when pedal bleeding to help fluid pulse back into the master cylinder....the fluid should squirt up almost out of the cylinder upon pedal return once all air is bleed out of the system.

[desoto1939]

I had a similar issue. The wheel cylinders were all good. One day over the winter checking the brake the brake pedal was fine. two days later the pedal went to the floor. Started to look at all 4 wheel to see if brake fluid was leaking but no leaks. Also pulled all drums to male sure.

 

I was able to pump the brake pedal and pressue came back then it went ot the floor again.

 

Pulled the MC out of the 39 Desoto which is under the floor board and a pain to take out becasue of the lackof room.

Tore the MC apart and rebuilt it with all new internal parts including a brass piston.

Did a bench bleed and put back inthe car and brakes worked fine.

 

Discovered that the  check valve at the front on the MC which has a rubber gasket inthe valve had a slight tear inthe rubber so this is what was causing the fluid to not have any pressure and the fluid would just bypass this in the MC.

 

recheck all of your internal parts of the MC. Also back off the brake shoes so you do have some movement when bleeding the lines.

 

Rich Hartung

desoto1939@aol.com

[ozzmonaut]

Thanks for the help. I tried bleeding without the shoes tight at first, but once I realized I was getting nowhere, decided to tighten them. I did not really think about the return. I just bench bled the new MC and got it put in. Going to wait til tomorrow to bleed the system again.

[desoto1939]

Also backoff the upper adjuster so that the shoes can move and the internal pistons in the cylinders can then come back to a normal position.

 

Letus know how you make out with the new MC and the brake adjustments.

 

Rich Hartung