So I have replaced the master cylinder, power booster, and both rear wheel cylinders. The car is a 1971 plymouth roadrunner with an aftermarket power front disk swap. (disks up front drums in rear). The kit uses what I believe to be a somewhat universal booster and GM master cylinder. The rear brakes are stock 8 3/4 drums. The pedal doesnt feel perfect in drive, but I havent had a problem with it being too soft, or having a lot of pedal travel. But the strangest thing happens when in reverse. Typically when I first get in my car and back out of the parking spot the pedal goes to the floor when I press it after ive moved a few feet. It does stop the car so I know it is applying some pressure, but I have to push the pedal extremely far and the pedal is very soft.
And when I put the car in drive and push the pedal twice it comes back up and the car stops fine and the pedal feels tons better. I checked the rear drum adjustment and the shoes are pretty tight against the drums. The master cylinder doesnt appear to be missing very much fluid at all. So I checked under the car, master cylinder, proportioning valves, steel lines, and rubber hoses, and I cant find any puddles or anything signifigant except some somewhat "wet" looking spots directly beneath the rear bleeding screws and around the connections on the rear wheel cylinders. Could air be seeping in through these spots and causing my symptoms? Anybody have any suggestions?
The steel brake lines were replaced with stainless lines 5 years ago, as well as the rubber lines.
Im thinking about having someone pressure bleed it with the master cylinder lid closed to find a leak if there is one. As always I appreciate any knowledge, experience, and helpful tips others have to offer. Full time college student working on this in my spare time.

that is odd.
the only thing I would think to do is run it in reverse apply the brakes only the once for it to go to the floor and then take apart the drum and see if the shoes shifted.

it definatly sounds like air,it doesnt take much.mabey the reason its worse in reverse is because once youve backed up the brakes are pumped enough where they work ok in forward.there should be NO wet spots anywhere.

tighten the connections that are "wet" till they are dry & just pushing the pedal will give you enough psi to confirm that they are now sealed up. then bleed the M/C then the 4 corners in order then holler back with where you're at. EDIT After rereading your post, after you get out of the parking lot they are sorta OK? I'm thinking the M/C may be bad but k.i.s.s. stop the leak(s) & rebleed & go from there

"when I put the car in drive and push the pedal twice it comes back up"
You definitely have air in the system. In reverse you also lose the self-energising action of the rear drums. What you're doing is pumping the air to compress it similar to the method for bleeding. The air causes the spongy feeling because the compression isn't permanant. Bleed it some more.
Ron

Thank yall for the fast responses. I havent had the time to perform the work myself in the past couple days so the car is going to a shop ive used before, tomorrow afternoon. I'll keep yall posted about the progess. Can you please elaborate more on the self-energizing action of the rear drums? Is this why in drive the front disks are doing the majority of the braking and I cant feel the spongyness as much? Because the self energizing action doesnt work in reverse and the air in the lines in the rear brakes is much more noticable?
Thanks again!

Drum brakes have a natural "self-applying" characteristic, better known as "self-energizing." The rotation of the drum can drag either one or both of the shoes into the friction surface, causing the brakes to bite harder, which increases the force holding them together. This increases the stopping power without any additional effort being expended by the driver, but it does make it harder for the driver to modulate the brake's sensitivity.

I think the fronts have air, are they oriented proberbly {sp}?

--Daty

So after returning the car to the shop to have them vacuum bleed the brakes, they told me that they got all the air out and that all connections were checked and tightened, and that the master cylinder was bad. The mechanic said that the master cylinder was allowing the piston to blow by. So the kit that I purchased is the Jegs power brake conversion kit, and to get another master cylinder I have to order it through them so I will have one here on wednesday. Also, they couldnt tell me what year model or part number so that I could get one at the auto parts store because their supplier makes them somewhat custom? But Jegs agreed to cover it under warantee. So I will keep yall posted on what happens next. Hopefully Ill have my car back by next weekend as this is my only car and I depend on it for my daily transportation. Thanks again guys!

So I went to pick up my car today and the brakes were still soft. That's with a new master cylinder and the brakes have been bled, and the rear wheel cylinders are new also. I do not know what else to check? Is it possible that I have the wrong rear wheel cylinders? I'm going to see if the mechanic can vacuum bleed the system and check for any leaks. Any ideas?

Have the shoes been adjusted out to the drums? This is one reason I like factory stuff.

The master cylinder is brand new, and has been checked that it is good. The rear drum clearance has been checked and it is adjusted to be slightly grabbing. As far as the distance from the pushrod touching the master, that has also been adjusted to be very close. Could that be causing a problem? After driving it for awhile it feels sometimes like the pedal is very hard and stiff at the top with not much pressure needed to stop the car, while other times its soft with more pedal travel needed. Any more ideas? I agree about using factory stuff. The master and booster with the brackets they came with are the only non factory items I've used.

Do the front wheel bearings need to be adjusted? If they are loose the movement of the rotor will push the pistons into the caliper, like when you replace brake pads and you have to press the pedal a few times after pushing the piston all the way into the caliper.

How could I tell if they needed to be adjusted? After driving it some tonight it seems very unpredictable as to when they will work and when the pedal travel is extremely far, and sometimes the pedal travel is short, but no matter how hard you push it struggles to stop the car. I am getting so confused by this. I'm open to all ideas

on mopar cars mastercylinder the front line goes to the rear brakes and the rear line goes to the front brakes----chevys are opposite--front line(master) go to front brakes---i would check the lines to be sure they have not been crossed up...bob

Ok so this is a strong possibility because I know the shop built custom lines for the new master cylinder. I think they might be crossed, but how would I know for sure? I see they go from the master into a distribution block right below and how can I tell which line goes where?