I did a search of the posts on the board and didn't find anything similar to my problem. I've got a 40 plymouth business coupe, and I'm trying to figure out why the brake pedal won't stay solid. It's got a brand new master cylinder...I can't tell you how many times I've bench bled it. new lines throughout, wheel cylinders were new when I bought the car. When I bleed the brakes the pedal will firm up and feel fine. I think all is good...until I come back in a couple of hours and the pedal goes to the floor. It will firm up with some pumping but not like it was after the bleeding. I am at a loss, and need some help. the brakes are really all that is holding me back on mechanicals as far as having this car ready to roll. any suggestions?
What is your brake set-up (stock or ???) Is the master mounted below the level of the wheel cylinders/calipers (like under the floor)? If so, you may need in-line residual check valves.
it's a bone stock setup. pedal is connected directly to master cylinder which sits on the frame rail.
Sounds like you are either airiating the fluid by pumping too fast and after it sits for a while, larger bubbles form. Or most likely you have a leak in the system. If the stock RPV in the master cylinder is removed or junk, air can seep past the cups in the wheel cylinders. In annother scenario you may also have scared bores in your wheel cylinders allowing a bit of air to invade when you release the pedal. Under pressure the cups are pressed out to the sides of the bore and no fluid would escape. Again as with other suggestions, make sure the pedal is returning all the way and there is free play between the pedal push rod and the master cylinder.
What the Polack said, plus: IS it possible that the cup expanders weren't put in the wheel cylinders?? I understand you didn't do the wheel cylinders, so this is a possibility. WIthout pressure of some sort, air will get in around the cups, esp. if the master is below the wheel cylinder level. Cosmo
Thanks for all of the help guys. looks like i'll be taking the wheel cylinders apart to start and then we'll see from there. i just remembered i never introduced myself. i'm from nashville, tn. own a 1940 plymouth business coupe. have owned a 1968 impala, 1960 ford starliner and some other cars in my short 31 years. this mopar is the oldest so far and i'm looking forward to getting it going soon.
good info on the Plymouth Owners Club about bleeding correctly ... Push only half way down ... make sure all 4 sheels are adjusted ... seems to have a hard time bleding because of the rear-end hump .. a air pocket is hard to remove unless you use a pressure bleeder on the master .... some say ... Mine I had no trouble bleeding the regular way... Good -luck
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