brake pressure point changing

[Cpt.Fred]

i have another small question, this time about the brake system.

on the first tour i had to stop with a smoking front brake, and i found out that it was the push rod between the master cyl

and the brake pedal that had changed its lenght, at first i thought because i didnt properly lock the nut.

i instantly changed that.

the next day it happened again (though i am 100% sure that i properly secured the nuts),

only this time i felt it coming soon enough and i changed the pressure point of the pedal to a lower level.

since then, it is stable.

i am not comfortable with the current situation, i'd like to have the pressure point higher,

so that i don't have to push the pedal that far down and have better control of the vehicle.

when i talked to a friend on the weekend who used to have a '41 Coupe,

he instantly knew what i was talking about and said he had had the same problem with his brakes.

did you ever have this problem, too?

[TodFitch]

The 1936-48 service manual says this about the pedal travel:

(1) Free play ("A," fig. 1) in the brake pedal should be from 1/4 to 3/8 inch. This free play may be readily felt by hand and is the movement of the pedal before the push rod touches the master cylinder piston. If necessary, this free play may be altered by changing the length of the piston push rod.

Caution

Piston cup (16, fig. 13) must clear port ("B," fig. 13) when the pedal is in the released position.

(2) After the free play is taken up, and the pedal is slowly pushed for an additional 5/8 to 3/4 inch ("B," fig. 1) fluid should be forced up through the relief port in the master cylinder. If it does not, and the free play is correct, the master cylinder should be disassembled and checked for swollen cups or improper assembly of parts. The additional pedal travel ("C," fig. 1) of approximately 1 inch is required to move the brake shoes outward against the brake drums.

Basically, you will have 1/4+5/8+1 (1 7/8) to 3/8+3/4+1 (2 1/8) inches of travel in a properly set up system. The biggest single part of this is the distance that the brake shoes need to move. A single brake shoe misadjusted can cause the brake pedal to move much farther than you'd like. Having all shoes slightly misadjusted can cause the same thing (and this is what happens when they wear).

So set the master cylinder push rod per the manual then don't touch it again until you rebuild/replace the master cylinder again. If your pedal is low, adjust your brake shoes.

[Cpt.Fred]

all brakeshoes are adjusted (as good as i can with selfmade tools) and they all seem to work synchronal.

i also have a manual and did everything like it's said in there. somehow the travel of the brakepedal got smaller and smaller,

in the end the car was braking continously and the brakelights did not get dark anymore...

i need to get on a car lift and have a closer look to all the parts again.

[greg g]

make sure the pressure relief hole inthe bottom of the mastercylinder is clear. If its plugged it can cause the brakes to drag, lock up, or have a stiff high pedal.

[RobertKB]

make sure the pressure relief hole inthe bottom of the mastercylinder is clear. If its plugged it can cause the brakes to drag, lock up, or have a stiff high pedal.

You beat me to it, Greg. My thoughts exactly and it would certainly explain the pedal not releasing the brakes.

[Don Coatney]

Pictured is the relief hole with the pin sticking in it.

[Cpt.Fred]

hmm, that sounds interesting!

the master cylinder is new, also it looks completely different like the one in your picture, don, but who knows?

after readjusting the free play, everything seems normal now, but i guess i disassemble it again to make sure nothing is plugged.

[Jim Yergin]

You do have a different master cylinder. The one in Don's picture is for a P15 and mounts underneath the floor behind the pedals. The one on your car should be in front of the pedals under the steering column, right? In any event it should also have the relief port.

Jim Yergin

[BobT-47P15]

Are all your brake rubber hoses new?

My rear brakes used to lock up if applied hard because the fluid

would go thru to the wheel cylinders then not be able to return

fast enough to the master cylinder. The hose had separated on

the inside, even though it looked good on the outside.

[Cpt.Fred]

jim: you're right, that's where it is. i have a small service opening in my floor panel, through which i can adjust the pedal's free play. that's where i think the trouble is going on.

bob: yes, they're all new. i guess i must have done something wrong myself.

the brakes do not really "lock up" all of a sudden, it's more like the pressure is sneaking up on me. i tried to get like 1 or 1 1/2 inches of freeplay, that's very nice to operate and the brakes are really great then. several miles later i realised the free play is nearly gone and before i could find a nice place to pull over i already smelled the brake linings.

now it feels more like kicking a sofa cushion and the pressure point is about 2-3 inches above the floor board. it works great, the car got a 310 on both front wheels and 260 on the rear on the brake test bench (is that measured in lbs?)

but you have to kick real hard in the end. if i set it up like this, the pressure point stays where it is...

[Cpt.Fred]

... because i have the same issue once again.

guess i will now disassemble everything once again to check this hole you told me about.

i also guess when i hunted down this one i'll be able to disassemble and reassemble the brake system with eyes closed and hanging upside down:mad:

Edited July 9, 2010 by Cpt.Fred

[Cpt.Fred]

ok, i did it again.

i took out the master cyl,

drained and disassembled it and cleaned everything with an air gun.

then i took my old master cyl from the shelf, took it apart, too,

and started comparing all the parts.

here's what i found:

this is a picture of the inside of the old master cyl's reservoir:

and here is a picture of the same area

inside the new reproduction master cyl i got from andy b. :

now in my understanding, from what i could gather from the manual,

the exploded assembly drawing in there, and don's photos above, the hole on the left of the pics,

pointing to the car's front, is the release bore you mentioned earlier.

i stuck a needle trough it and blew air trough it to make sure it's really clean (it had been before...)

the other hole, however, just lets brake fluid enter the area between the

2 rubber seals on the piston.

though these holes have a rather big difference in diameter on the two master cyls, i cannot believe this could be connected to my problem.

what do you think?

again:

when i reassembled the brake system,

i dispensed with the glas reservoir i mounted on the firewall during the

restauration and used the original filler plug for the master cyl.

though filling and bleeding the brakes is a pain in the butt now once again,

i thought maybe i'll just "reset" everything to factory settings and then see what happens...

yesterday i made a test run across town, and everything seemed to work quite well and normal.

but somehow i keep thinking it's too good to last...

Edited July 20, 2010 by Cpt.Fred