1976 280C W114....Brake bleeding.....HELP!!!
 

I have replaced the master cylinder.
I have replaced the rotors.
I have replaced the pads.
I have rebuilt the calipers.
I have replaced the brake booster vacuum hose check valve.

I bleed the brakes starting from the rear passenger, rear driver, front passenger and then front driver's ......here's my issue.....When I do the fronts, I lose pedal pressure.
I cannot keep brake pressure.
It seems that I'm getting air in the system but where?
Everything is tight no leaks.

Could a bad brake booster be the culprit?

Thanks in advance.

Brake booster...no.
Did you bench bleed the master?
Fill the reservoir slowly and confirm using a light that both halves are full, the rear half fills a lot slower.
If all else fails get an inexpensive pressure bleeder, they work well and are great for flushes.

Good luck!!!

I agree - I would suspect the master cyl. as well.

Agree. Bench bleed the master cylinder.

If your abs as air in side it , then it will be very hard to clear it .

I apologize, I wasn't clear, this is on my 1976 280C (no ABS).

Not to drop any brand names but I use a vacuum hand pump made by a company called Mighty-Vac. It can not only be used to bleed brakes and master cylinders but is useful for testing all sorts of vacuum related devices.

They make two types: A plastic body and a metal body. I have had better luck with the plastic body types. They just seem to last longer.

And if you ever own a 116 with the Chrysler based auto temp system you will have a lot of vacuum stuff to test.

And you can bleed a master cylinder in a few minutes with one of these things. They come with, or they once did anyway, a plastic cup that catches the brake fluid.

Make sure you have not swapped left and right calipers on the front. They will fit either side. bleed port must be on top.

The simple way to bleed a master cylinder is to attach a piece of plastic hose to the MC bleed valve, fill the MC with brake fluid, and then bend the hose around to where it discharges into the brake fluid tank.

Then slowly pump the brake pedal. When the bubbles stop the air is out of the MC system.

1. Are any of the calipers mounted upside down ?

If they are then you'll never get them to bleed.

The bleeder valves must be at the top of each caliper.

Yes, fronts are upside down.
I'll get on it as soon as my pressure bleeder gets here.

Thank you.

Yes, fronts are upside down.
I'll get on it as soon as my pressure bleeder gets here.

Thank you.

That means you have to swap the left and rights to get the bleeder valve in the correct top position.

As BCBens was saying.

Over the years I have had incredible problems with are being introduced into the braking system at the bleed valves. I mean around the threads, not the center. If it absolutely is not closed when you raise the pedal back up air can come in the bleed valves even if you see nothing coming back in the tubing.

Well, I bought a Schwaben pressure bleeder and it is awesome. I bought it for $36. After I installed the calipers correctly, I pressure bled the system using 12psi and the pedal is perfect now.

Thanks guys for all your input and support!