Soft pedal .

Today I filled the all new brake system with fluid. I Bench bled the master cylinder using the tubes into the reservoir method and stopped once no more bubbles were seen. Bled rear brakes first with a mighty vac and then fronts. Figured out fairly quickly that my pedal rod adjustment was not correct and I had no play without brakes applied and front wheels were hard to spin. I corrected that. So now the problem is lots of pedal travel and a soft pedal.

Details
Manual brakes. Disc front, drum rear.
Rebuilt GM manual disc master.
Right stuff combination valve.
Adjustable proportioning valve in line to rear brakes.
All new components including rubber hoses, brake lines and calipers.

Using DOT 3/4 fluid.

Suggestions?

 

I may catch flames for this, but I've had success with just opening the bleeder valves on drum brakes and letting the brake fluid run out. They DO eventually bleed out. Gravity bleed is, of course, at a lower pressure than a bleeder ball, which is a shop tool for rapid turnover for profit. Discs of course need some encouragement. One thing- did you use a tool to keep the proportioning valve safety piston (drawing a blank on the real name atm, I used to know this **** cold) to make sure that all circuits are open?

 

Make sure the calipers are not binding in their brackets. I had a similar issue with the aftermarket front disc brake kit, where I had bent the brackets while trying to work on the ball joint replacement. This made calipers not square in the brackets and binding. The calipers would move when pressing the pedal, but not retract enough when pressure was released from the pedal, because they were binding in the brackets. I made some measurements then bent the brackets until the pads were same distance to the rotor in the front and rear of the pad. I also had to file the brackets a little in some areas so the calipers would move smoothly. Everything worked normally after that.

 

Is the prop valve opened all the way?

 

??, did you replace rear brake shoes too? I had a similar issue when I did fronts and rears and turned out to be rear shoes not adjusted properly which caused soft pedal.

 

You can carefully pinch off all 3 brake hoses and then see what happens to the pedal. Hard pedal = problem at one of the wheels. Soft pedal = air likely still in the master cylinder. Sometimes you can isolate front or rear depending what's wrong and how things feel by removing one clamp at a time.

I like to bench bleed the master cylinder using plugs in the ports rather than tubes.

It's difficult, but not impossible, to bleed brakes by yourself. There are cheapie brake pedal depressors. Pump the brake pedal with your foot, install the depressor in place of your foot when your foot is down, and go out to a wheel and open the bleeder. It's tedious by yourself but not impossible.

 

Update. Combination valve still centered, adj proportioning valve fully open. Rear brake shoes snug and both rear wheels drag. I suspect a faulty rear brake section of the master cylinder.

 

I agree . I always Bench bleed the master cylinder with tubes from the outlets back to the reservoir and then Gravity bleed each wheel ,

 

Thanks for all the suggestions. Turns out my master cylinder is bipassing in the rear half. Of course I bought this years ago and it's been unused on a shelf until now so no recourse and that particular part number (10-1479) is out of stock everywhere. I think I've located a suitable replacement.

 

Glad you found the problem and a replacement master cylinder. Years ago, I had problems finding a replacement manual disc brake master cylinder for my 74 Malibu, too.

 

That's the kind of stuff that stinks out loud. I've had components expire on the shelf, too. Good that you got it chased to ground.

 

So it looks like I jumped the gun when I ordered a replacement master cylinder ( ok fine it was 3) when I disassemble the first one that was giving me a soft pedal it looked fine inside. No pitting or rust, seals and cups were pliable. So I put it back in the vice and tried bench bleeding it again. Front section was easy to bleed but the rear took a lot longer but it did bleed properly. Installed it on the car and pedal is much better. Once I do a final bleed on all 4 corners it will be good to go. As a bonus I now have 3 extra new 1" bore master cylinders

 

That last little bit of stroke can be tough to get when bench bleeding...