losing brakes

Hello

I'm having a wierd issue with the brakes on my 78 PW. when I start moving in reverse the brake pressure goes away and the pedal goes to the floor. I would think that this would be a master cylinder issue, but I have put a new master, brake booster, right caliper along with new rotors and pads. I can't seem to find any leaks at all. Do you think that the new master cylinder is bad? It was doing this before I replaced all the brake parts, along with a sometimes hard pedal and sometimes loss of pressure. That's why I replaced all these parts in the first place.

The vehicle is a 78 PW W200, 440 auto.

Any input would be great

Imho. Anytime the pedal goes to the floor there is a lot of fluid going somewhere.

If it only happens when backing I would first check that the rear brakes are the correct diameter.

Without adjusting the brakes see if you can remove the rear drum.
Then measure the diameter of the brake shoes. And the diameter of the drum.

My thinking is in reverse the self energizing action of the drum brakes is not as good as in foward. Maybe the shoe diameter is too small and the displacement of the cylinders exceeds the stroke of the MC.

Something in the rear brakes or e-brake.

Or the caliper you replaced is the wrong one. Calipers on wrong sides throws bleeders to wrong position. Seen this before. If the bleeder is on the bottom you wont bleed all the air out of it unless you take caliper loose and flip it over to bleed it.
The pedal drop would come and go with pumping brakes regardless of direction your rolling.

Did you bleed until clean all 4 brakes? starting with the pass rear, drive rear, pass front, drive front

I agree if front disc and rears are drums, then something is going on inside the rear drums that is not supposed to, And I think Dana67Dart is on to something.

alrighty then, I will start with the rear brakes.

I replaced the caliper, booster and master. I had a shop do the rotors, pads and soft lines. I needed new studs pressed in as well.

Thanks all for the input, ill get on this tomorrow.

Sounds like somebody put the brake shoes on backwards.

Ok, well I went through the rear brakes. They were a bit out of adjustment so I feel like I got them straightened out. However that did not solve the problem. I noticed that when I am backing up AND turning the steering wheel to the right the brakes fail (lose pressure) and the pedal goes to the floor. I am still not seeing any leaks. If I turn the wheel to the left while backing it does not do this.

Any other suggestions?

Well I can tell you in 63 years,64 tomorrow, I have never encountered what you are describing.

That's not encouraging!!

Do the trick to get the pedal to fall,but do not stop. Then, while still backing and turning, pump the brakes to see if the pedal gets hard.
If the pedal gets hard; then this is a mechanical issue. Something has knocked the piston back, or the pad is not anchored properly or the pads are stuck in a rut in the caliper ways; Ima thinking this last one. Now,this turning and backing thing, has released the caliper, the pad jumps out of the rut, moves into a proper braking position, and you feel it as a drop in the pedal, which you interpret as a pressure loss. Pumping the pedal pushes more fluid out to the caliper and restores the braking action, and pedal-hardness.
But if the pedal does not get hard, then IDK, but here are a couple of other checks;
Check your front brake hose routing, just because, not that I think there is an issue. Next check your lower control arm bushings are not floppy.
If this is OK then next check your front wheel bearing adjustment; With the wheels off the ground, and the steering wheel sorta centered; give the lowest part of the tire a solid kick, and a hard tug.Hit her hard, you can't hurt it, you'll break a toe first..

Finally;Check your power booster operation
turn the engine off, and pump the brake pedal about 5 times, to empty the storage chamber. Then,in Park, press down and hold the pedal while starting the engine. As the engine idles, the pedal should fall about 1 to 1.5 inches, within just a few seconds..
Do this at full-lock left, and at full-lock right; It should drop the same.Steering wheel position should make zero difference. This is just to rule out a booster issue.
That's all I got.

If the caliper ways are rutted, you will have to fill them and dress them down to fit. I use a hard brass brazing rod. Make sure the caliper pins are not bent and move freely in their sockets.

OK, I will get back at it and see what I will find. Thank you all again.