Any brake geniuses out there?

[51 MERC-CT]

optimum word expand, do drums expand into the shoes or away from the shoes???

The diameter of the drum will get bigger/expand when heat is applied (from brake shoe friction).
I suspect that brake shoe material has a coefficient of expansion that is close to Zero.

 

[Kurt in NJ]

what did you use for fluid in the brake system?? ---fluid contamination can cause the rubber cups to swell up causing your problem----when the brakes are locked up loosen the master cylinder bolts ,if they release it's the pushrod, if not look in the master cylinder

when you say "hydrovac" do you mean the booster run by the power steering pump? ----or is it a seperate vacuum booster plumbed into the line after the master cylinder (servo booster)

 

[V8COOPMAN]

The diameter of the drum will get bigger/expand when heat is applied (from brake shoe friction).

This is one REALLY good reason NOT to turn drums any more than the recommended dimensions, like 12.060" limit on our old Ford drums. The more meat in the drum, the less likely it is to distort in diameter. DD

 

[1ton]

My humble opinion is that heat is the problem. Whether it be from something dragging or heat from the exhaust near a brakeline. Any heat is going to expand the brake fluid causing tighter braking. Since yours is a new problem, my guess something is hung up.

 

[Kahuna]

Maybe something to do with the Hydrovac unit, but I really don't know the internal workings of them

 

[Tinker]

It all started with a pair of leaking wheel cylinders on my '47 Marmon-Herrington/Merc woodie, equipped with a Chevy 350 and Hydrovac booster. After battling with a bunch of repro crap and several adjustments and system bleeding, we ended up with 4 good cylinders, properly fitting shoes, new rubber hoses, new master cylinder and a firm pedal with the right amount of free play.
It all worked fine on the way to our local C & C today, but on the way home - about 5 miles - the pedal got higher and higher. Finally back home, there was about zero free play, the brake lights were stuck on, and I couldn't budge the car with a push. After about an hour, I checked it again and everything was back to normal.
I suspect something blocked or stuck in the booster, since it's the only component we didn't replace...any ideas?

any update!!! 7 can be a lucky number being your post num to this point.


Sounds like fluid expanding under heat or.... that sbc! ha ha ya I do that.

 

[Mart]

1ton said it is heat related. that's true. All brakes get hot when used. the fluid warms up and expands. When the brakes are released there is an open port (the recuperation port) that allows fluid pressure to equalise between the system and the reservoir. When you press the pedal, the very first part of the travel closes this port, allowing you to pressurise the system and apply the brakes. you might sometimes observe a little squirt of fluid into the reservoir as you apply the brake pedal, that is coming from the recuperation port.

If the pushrod is too long the port is already closed and the fluid can't equalise before the brakes are applied. The heat from the brakes further pressurises the system and applies the brakes even harder. I't's a vicious circle.

If the pushrod is too short, you get a lot of lost motion before the brakes are applied.

The trick (and I have done this) is to fine tune the pushrod length so you minimise the lost motion but still maintain an open port at rest.

the other important thing is to ensure the brake pedal has a good return spring so it returns to the up position cleanly. If not, you could have a similar scenario where the recuperation port is not open.

Mart.

 

[barnfind]

Very old rubber brake line hoses can act as a one way valve due to internal swelling.

 

[TJ]

Very old rubber brake line hoses can act as a one way valve due to internal swelling.

In the initial post he indicated he replaced all components including new hoses.

 

[Kurt in NJ]

In the initial post he indicated he replaced all components including new hoses.

I have seen new hoses that work as one way valves--- I have seen new wheel cylinders with holes drilled in wrong places--- if the edge of the cup covers the "in" hole just enough it can prevent return of fluid, I have had new master cylinders with the pushrod hole not deep enough, holes incompletely drilled and rubber that swells in brake fluid

 

[Kahuna]

Hydrovac units are powered by vacuum. Maybe it's located close to something & gets hot & upset?

 

[George/Maine]

Those rubber hoses will do that.
What some people do is pinch the line and this is NOT Good.

 

[rich b]

Wide range of ideas here; but the original post says the brake lights were stuck on; kinda indicates the master cylinder was holding pressure; either by defect or misadjustment of the push-rod, most likely cause as previously mentioned.

 

[mh_merc]

I'm getting closer...there seems to be a fine line between push rod adjustment and brake shoe adjustment that needs to be balanced. I should add that I've been driving this car for over 25 years; the problem only came up during this brake job...after finally getting new wheel cylinders that didn't leak, brake shoes that fit properly (relining the originals was the answer), and a U.S. made Wagner m.c. that didn't have to be pumped up for every stop. I appreciate everyone's expertise, experience and willingness to help!