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I've been through these rear brakes on this Camaro rearend now for the third time. I may still be blowing wheel cylinders, but whatever the problem is, I cannot get the rears bled, and my pedal is still going to the floor. Why would I keep having bad wheel cylinders?

Couls be the prop valve. Sometimes they'll hang up, especially if a wheel cylinder blows. Sometimes a light tap on it will free it up.

With both blowing I doubt if it's the cylinders.

Despite my frustration I just spent another hour going over the brakes. I can find no leaks whatsoever along any of the lines. I've tightened all connections, and my new wheel cylinders appear to be holding. Doublechecked and tightened the line connections at the master cylinder. Rear drums are adjusted so that they are dragging just a little, so they should be close. Pulled out the Haynes Camaro manual again and went through the instructi9ons on brake bleeding. Paragraph 8 reads, "On vehicles equipped with a combination valve the pin in the end of the metering part of the valve must be held in the open position. This can be carried out using the official tool or a similar device clamped under the mounting bolt which should be temporarily loosened. The pin must be pushed and held in."

Pardon my ignorance, but what is this all about? I've worked on a lot of mustangs with disc brakes over the years and never heard of this. Can anyone enlighten me about this. I really need to bleed these brakes and wrap up this issue. Which end is the metering valve portion and where do I find this pin they describe? What does the Official Tool look like and what can I substitute? HELP!!!!!!!!!!!!!arrrgh!!!!!!

BTW, what are the symptoms of a bad combination valve?

Don't know if this will be any help but......I've had more than my share of problems with trying to adapt factory combination/proportioning valves to project vehicles. Without going into a novel about the problems I encountered, I'll just cut to the chase and tell you how I set up my systems now.

Dual master, adjustable proportioning valve (Summit SUM-G3905, $35.95) 2 (if using front discs) & 10 lb residual valves. The prop valve goes after the 10 lb. valve in the rear circuit. You've got your rear shoes dragging slightly.....open the valve up completely, bleed all circuits, crank valve down to full closed and back off 2 turns for road test. Don't forget to readjust your rear brakes. You may have to make final tweaks to the adj. prop. valve depending on your particular driving habits.

The "pin" that was used in FoMoCo valves required a little clip that would hold it out during bleeding process and I've had to remove the guts from other valves to get them to bleed. The adjustable prop valve will eliminate your problems.

Dave

For posterity's sake, I want to let everyone know that the pin at the end of the combo valve must be held in the depressed position to bleed the brakes on the Camaro front disc/rear drum setup. I finally have success and brakes. PRAISE GOD!

I found this last night on the net at www.nastyz28.com :

"RRD, get a small 90 degree braket or make one. place the brack on the proportioning valve so that the bracket holds the pin on the end of the valve in and will not let it move out. Use a vice grip plyers to hold it place. Then start to bleed your brakes. Start with the wheel that is farthest from the master cylinder, then the next farthist and so on. You do not have to push the brake peddel to the floor or push it to hard. Just use your hand to push it in 1/4 to 1/2 or untill you fill pressure against your hand. (you may not get much pressure at first or fluid). Then hold it in that position. Have some one help you to bleed the wheel cylender. Make sure that the brake pedel does not move out at all untill the bleed valve is closed. Go esey at first. You may not feel much pressure at the start just keep working around all the wheel cylenders. Make sure that the master cylender stays full don't let it get to low. After you get fluid from all wheel cylenders and a good feel on your peddel you can take the bracket off and see if the pin does not move. Make sure that the master cylender is bleed befor you hook the lines up to your brake system. I beleave that you may have pushed the brake beddel to far alowing the proportioning valve to over extend. Most of the time when this happens you will feel a click or pop in the peddel and the it does not feel right after that. The valve has blocked of the back brakes as if you had a leak keep the rest of your fluid in the front side of the system. In is made to work that way. Rember don't push your peddel to far or to hard, and don't let the peddel move out befor the bleed valve is closed. Your braks will come back."

Good call! I was about to jump in with the same suggestion, but you discovered the right info first. A proportioning valve is there to allow the rear brakes to apply slightly ahead of the front to avoid front wheel lockup during slippery condition driving like on ice or wet snow. The "combination valve" that most dual circuit master cylinders have serves several functions, including isolating one end or the other of the system in case of a pressure loss. It's necessary to disable that function if there has been a major fluid loss, like replacement of both rear wheel cylinders, for instance. The valve interprets the total lack of pressure in one end of the system as a catastrophic failure, and isolates that part of the system.
Jerry

As mentioned previously the pin will set in the proportioning valve and shut off the rear because it senses a failure and is doing it's job. I found a fairly simple solution when installing all new parts and all the lines & cylinders are dry. I use a small pump type oil can with brake fluid in it. Remove the line from the prop valve outlet, crack open one wheel cyl bleeder, pump the line full, close the bleeder and do the other side. Hook the line back up and bleed the system. It saves a lot of hassle.
Fred