Technical Hard pedal, manual drum brakes

‘65 Pontiac Catalina. Stock single pot system. Rebuilt MC, new lines, new wheel cyl, new hold down kits, drums and shoes. What happened: I rebuilt the MC since it’s what I suspected wasn’t working. Brakes would feel normal after bled, I could pump them while sitting and the pedal travel would decrease some, but not get to nearly zero. Once the car moves, subsequent braking action reduces the pedal travel. I took my car out for a short test drive, when I left the pedal was good, I returned and the pedal was bad. I had to crack the main line off the MC to release the pressure, so the car would move. Observed rear brakes were warm, and the fronts were hot to the touch.

I think I can eliminate the MC as the problem. New rebuild kit and it feels normal until the car moves. I suspect the front shoe springs or something in there. It’s as if they won’t fully return to neutral without help by reducing the excess line pressure at the MC. It should have enough force to compress the wheel cylinders and force the fluid back into the MC. It seems to do be able to compress the wheel cyl and push the fluid back into the MC after freshly bled, but something prevents that from happening after the car moves and brakes several times.

I know this is one of those frustrating issues others have experienced that has no clear solution.

I’ll break down the front drums and check that the pedal rod has a little slack for proper return. Not sure what would help. Maybe the shoe return springs are weak. They aren’t brand new but worked fine when last used on the car.

It can’t be that complicated, it’s a fairly simple system. Any thoughts are appreciated.

 

check the rubber hoses ..been posted here that if they are new old stock , they deteriorate from the inside ....only other thing i can think of is proportioning valve...it gives more pressure to the front brakes to compensate for forward inertia....keep us posted

 

If the flex hoses were collapsing, it seems logical that there would be some noticeable pedal resistance while bleeding at the wheels. I can say that pedal feel while bleeding had no resistance.

 

X 2 on bad brake hoses. The inside comes loose and acts like a one way valve. lets fluid out to the wheel cylinders under high pressure. But will not let the fluid back to the master cylinder under spring pressure. I have seen this many times.

 

The deteriorating rubber acts as a one way valve . 3 of my 4 vehicles have had this occur .

 

If it wasn't doing this before the mc rebuild, I would think that maybe there was a problem in there. Maybe something to do with the pushrod, like it's not seated properly.

 

I resurrected an older motorcycle that had brakes tighten up as I rode. I could crack the bleeder on the offending caliper and things were fine until I used the brakes again. That rubber (flex) line was collapsed inside acting as a check valve. When the brakes heated up the fluid expanded and was applying the brakes.

As far as not having unusual resistance during bleeding, the pressure you apply through the pedal, multiplied by the pedal ratio, far exceeds the pressure that returns fluid to the master when releasing the brakes. I'd suggest starting to check at the wheel(s) that heat up the most when driving.

Lynn

 

Verify the brake pedal is retracting completely. A weak return spring or too-tight push rod can keep the master piston from retracting completely, causing pressure build up when driving.

 

Free play in the brake pedal ?

push rod adjusted correctly ( free play ) ?

brakes adjusted correctly not to tight ?

flex line collapsing ?


Are all brakes under pressure ?
Only fronts / rears ?


If everything is rebuilt correctly I would start at the pedal / push rod .

 

I'm casting a vote that says it's the push rod length causing the issue. It's to long. It isn't retracting enough to release the pressure. Brakes eventually locking up is the result.

 

Okay, you are saying that after a drive the pedal is lower and the brakes don't release, right? If cracking the line at the MC relives the pressure, then the problem is with the MC. My guess is that the piston isn't moving far enough back to open the compensating port thereby not relieving system pressure. The most probable cause is that the push rod isn't allowing the MC piston to return all the way.

 

You said you re-built the MC. Was the new piston identical to the old one? Lippy

 

It’s all stock OE parts so no mix and match. But everything will be rechecked.

 

yes

 

The pedal travel decreases to nearly zero and I believe it’s coming all the way back up but I will recheck that.

 

Acts like a check valve. I like that. OK. The hoses are old NOS. When you start buying NOS parts 20 years ago that could already be aged, it possible the insides are collapsed.

 

Why would it only act like a check valve after the wheels have moved. Pedal operates normal after cracking the line to relieve pressure. I’ve pumped them 50 times and it doesn’t change much but once I roll and stop repeatedly the pedal travel decreases and get super hard.

 

Have you checked that there is free play in the pedal? If the master cylinder is not returning all the way, then the brakes will eventually lock up, and opening a bleeder will relieve the pressure.

Check that the push rod is not tight when your foot is off the pedal. Easy to check, easy to fix, usually the culprit.

 

NoelC makes a different point that could be valid. The idea though of holding onto and using old flexhoses is not healthy. The the too long push rod is a real thing but flexhose deterioration is much more common, and easier on the back to correct. I have done so many times

This thread was edited by removing quotes from other members.
my senses were jogged yesterday. And I apologize for being coming on too strong. I felt I could save MattK22 some time with a strong attitude. My bad.
I have, in the '70s , Had my sister in law's Fiat do the same and shortened the push rod. I regret being a bull.

 

I'm with Jim, I just figured he had checked the pushrod adjustment, hence the piston question. Lippy

 

For the record, I knew about collapsed brake hoses, but I’ve never had it be an issue in the past. Second, my understanding was that if they collapsed, they didn’t flow well, or at all, in either direction. Never heard anyone say they act like a check valve until the multiple replies in this thread today. All good info. It makes sense tho. The master can push with much more force outward than shoe return springs pushing fluid back to the master. So next move is to replace the hoses.

 

Did you check the pedal for free play? Might do that first....

 

It would be cool if Jim is right. Save time and money.
We 'll wait to see. I get it from, Jim's angle having changed MCs many a time.

 

I had exactly the same symptoms you did, though the rears would release enough to drive after about 30 minutes. 2 different things at play…first was the old flex hose to the back brakes was acting as a check valve…second was a small return port in the master cylinder had a partial obstruction. Hope this helps.

 

If it were a flex hose problem, cracking the line at the master cylinder wouldn't fix the problem.

A friend of mine is freshening up a 57 Crown Imperial to sell that his dad restored maybe 20 years ago. He was having a rear brake problem in that they wouldn't release. He borrowed my hub puller and fooled with the rear brakes for a while before he told me his real problem. He only confided in me after he got everything back together and had no brakes at all, but a hard pedal. We verified that the booster was working so I told him there had to be a blockage probably in a flex hose. After a little fooling around with the pedal we managed to get all the brakes locked up. He already knew that loosening a bleeder screw would release the brakes. My suggestion was to start at the master cylinder and loosen one line at a time until the pressure released. This car has a really strange brake setup and some of the fittings near the master cylinder are hard to get to. It also has two short flex lines between the master and the block going to the wheel cylinders. Both were plugged solid. New lines, good brakes.

 

Stop and think about what you did to release the pressure. You loosened the line at the master. Therefore the problem has to be upstream of that fitting. The only thing upstream of that fitting is the master cylinder. Like other people have told you, check that the pedal/pushrod is returning far enough. If it's not that, it's internal in the master cylinder.

 

Bigdog you said it right. I was wrong this morning. Had bad Hose in my face.
Justoutside under my sister's car I realize how stupid I was being. VANDENPLAS, NoelC, squirrel, lippy, and MattK22 I apologize.

 

Did the rebuild kit come with a new Residual Valve? If it did, it may be malfunctioning. Do you still have the old valve to swap back and see if anything changes? Do this after verifying the push rod adjustment. Just my 2 cents worth, and usually valued at slightly less than face value...

 

Did you happen to compare the new m/c parts with the old parts? Lately I’ve become a bit gun shy from too many wrong parts in the right boxes

 

Yes. I compared. Guts looked good. The snap ring was wrong. It works great. My bet is on bad front hoses.