Big Dummy

Brakes are crushing me. 1979 C3. New everything, except proportioning valve. New Proportioning valve seal failed so I installed old one again. Bench bled master cylinder. Vacuum bleed then pedal pump method. I can get rear calipers to bleed and have a good pedal. As soon as I attempt to bleed front caliper, I lose pedal and it won't pump up. When the car sits a day or two pedal is back!. Bleeding will cause same condition. I have tried two new master cylinders thinking maybe I hurt them bench bleeding, but same problem. New Power booster checks out OK - I do not have a brake light problem. What am I doing wrong? As simple as trapped air? Bad prop valve? Any light on this issue would be appreciated.

7t9l82

its not really a proportioning valve, more like a switch. open one front bleeder and depress the pedal close the bleeder. that should center it. i always gravity bleed the system. jack the car up loosen the front bleeders and back inner put some rags on the floor to catch the fluid . keep the master cylinder filled, it may take a while . as you get steady fluid close that bleeder. you need to be aware that one or more hoses could collapse inside and allow no flow.

Big Dummy

Thanks for the response. I'll try that as soon as I can. Probably next weekend. Mark

mrvette

There is a whole long list of the mods I have done to my brakes on the '72, one of them was to take out that silly switch/valve, and run the back brakes to the m/cyl directly, same with front, and use a divider block for the two front lines, same as in rear.....

7t9l82

it is much nicer now days because i can work on things as i want to and not have to worry about driving it to work tomorrow.

Big Dummy

...an idea I'm open to. Where to get them? I see some on the Muskegan Web page, they look like I would need to modify the brake lines. Any more info?

noonie

Google for Weatherhead brass fittings. You'll have to search by all the tube nut sizes.

Much easier to just take the existing valve apart and disable the proportioning valve part.

Sometimes just a good cleanup will do.

In reality, since you can bleed your rear brakes, I doubt the prop valve is your problem anyway.
Try pressure bleeding the whole system first.

Here is a pic

Big Dummy

Hello noonie - I have used a hand vacuum pump and I borrowed my friend's pump that is hooked to my compressor. Vacuumed around bleeders two times in GM's order, then proceeded to manual bleed procedure. That is where I run into the problem described above. I did note that on last attempt with power vacuum I could not get it to draw from front left, but I could get some fluid with hand pump. Anyway, that is where I lost pedal. I was looking at pressure bleeders, do the work better than vacuums for this system? Good idea to make prop valve a distribution block. Switch Piston and Prop Piston are the parts removed?Thanks for your input.

Jeff_Keryk

Motive Bleeder worked for me. 1st time I ever had pedal.

noonie

Pressure bleeding, not vacuum, is the standard and works very well. Simply, you need enough fluid flow at one time to push all the air bubbles out ahead of the fluid.

To gut the existing valve, the detent spool separating the front/rear is left in, just clean it well. Use alcohol.

The prop valve for the rear is gutted and the bleed hole with the small rubber disk is threaded and a brass plug installed. It's a little tricky to take apart the prop valve, but if you gently heat it, the pressed together parts will separate rather easily. See lower right parts in pic.

There is also a cheap little tool you can get that replaces the switch in the differential valve part that locks the spool in the centered position for bleeding.
Should not be needed for pressure bleeding, just when using the pedal to prevent tripping it, thus blocking one side.

Big Dummy

...so bottom line here is try the pressure bleeder before messing with the proportioning valve; do I understand you men right?

leadfoot4

Correct!

Big Dummy

..thanks for all the input. I just bought the Motive from Summit. My weekend task is set - weather permitting.

noonie

I would take it apart and make sure it's clean and centered before wasting any more fluid.
Not a big deal, you've already had it out, so the tube nuts are free.

Big Dummy

OK, sounds good...how did you know I have wasted quarts of fluid - ouch.

doorgunner

I'm paying attention----I have the same problem...thanks for all the tips!

TOM B1

Make your life easy, go to your local parts store and get 4 self bleeders. don't need any vacuum or pressure pumps, my wife hates helping and I don't like to buy tools I don't need.
I use them on all my cars.
start by opening up the right rear bleeder, pump the brakes a few times slow. next is the left rear then the right front then the left front, check the fluid after each bleed. your pedal should be good and the brake light off, if the brake light is on then rebleed the front brakes.

gcusmano74

Your clue is that you cannot get fluid from front left; change that hose.
Bleed the MC first.
Then gravity bleed the system, one bleeder at a time. Put maybe 10 sheets of magazine paper under each front wheel. Now you will be able to easily turn the front wheels back and forth by hand.
Next, jack up the rear of the car and remove the rear tires. Open one bleeder at a time, rears first. Wait for the fluid to flow clear. This takes time. Bring some coffee or whatever your favorite beverage might be.
You should have a good pedal when you are done.

leadfoot4

Keep in mind, a C-3's rear calipers each have TWO bleed screws.....you'd need 6 of the "speedbleeders".

Big Dummy

New everything includes brake lines. New parts can fail but soft lines are fine, no collapse. I can get fluid out of left front, that is where I lose the pedal. My friends compressor powered pump would not pull from there for some reason, I could get fluid with my hand pump. I already had 6 good quality speed bleeders and removed them because they didn't work with the vacuum pump, at least not for me on that day. My brother uses them on his bike and says they work great. There are a lot of opinions on bleed order out there, I have tried two. GM says LRI - LRO - RRI - RRO - LF - RF. Lots of people use RRI - RRO - LRI - LRO - RF - LF. Tap with rubber mallet too. Old School says gravity; I think they will all work if all is right. I appreciate all the information from everyone. The Forum has helped many times through this 2 year + project, I dig this place! Most of the time one can find an answer by just reading. I plan on giving back when I can. Mark.