grannys442

Car is a ‘69 442 with factory front disc.

On some vehicles, its common to insert a small threaded tool/fitting in place of the pressure differential switch while bleeding brakes.

Photo of differential valve in question. Should the electric switch be removed and replaced with tool to immobilize internal valve during the bleeding process. Switch has 7/16 x 20 thread and am having hard time finding tool of that size.

Questions:
1. Is it advised to do this on the differential valve of the car in question? If so, I’m having hard time finding that tool with a thread size of 7/16 x 20. All i see online are 1/2 x 20 and appear intended for use with “GM proportioning valves”.

2. Is it possible to carefully bleed the system without this tool and not cause the valve to go off-center (resulting in poor flow to rears) ?

I’ve recently replaced/rebuilt most if the brake components and the system is empty (with exception of MC which i bled on bench).

Thanks,
Adrian

Orlando 1

I never heard of this procedure. Double check your service manual. Normally I would bleed from the wheel cylinders and calipers or bench bleed the master. For the master there are plastic fittings that fit the exit ports and a couple pieces of tubing to direct the fluid back into the reservoir and a clip to hold the tubing in place. For the wheels the tool I use is a small bottle with some wire looped around the neck so it can be hung and no more than 18 inches of 1/4 inch clear tubing. I think drums use a 5/16 wrench and single piston calipers a 3/8 inch to loosen the nipple. Any shop manual has instructions for bleeding brakes.

grannys442

I see now that the switch in this particular Pressure differential unit has a fixed end rather than a plunger tip used on the various proportioning valves. Therefore, temporarily replacing this type of fixed-end switch during the bleeding process will likely not change anything. Essentially it would equate to replacing a fixed end (ie non-plunger tip) switch with a fixed end tool...so nothing gained. (???)


Phillips screw on bottom is not a plunger like those found on some proportioning valves. So my assumption is there is no tool/fitting needed to replace this unit during bleeding process.

I’ll wait to see if wise men chime in before i start bleeding the system.

Orlando 1

'Bleeding the system' can be done on a bi-annual schedule and does not require removing any lines as you describe here. Most people don't do it often and the fluid absorbs more water, corrodes internal parts and loses it's ability to resist compression. Is there more to the story here? What is wrong with your brakes that you feel compelled to pull apart the proportioning valve?

Reference:
https://classicoldsmobile.com/forums...ng-tool-93434/

grannys442

Thanks for the conversation and I should have given better preface.

The car was parked by its' original owner 30 years ago. I took possession of it a few weeks ago and am in process of bring it out of hibernation. Objective is leave it as original as possible, rebuilding rather than replacing parts, and ultimately making it road worthy. The entire brake system needed TLC...I actually destroyed one of the original brake drums getting it roll out of it's grave and onto the wrecker

My experience in these situations is to rebuild/restore the entire brake system rather than piecemeal. So I've rebuild, replaced and refreshed the entire brake system with the exception of the hard lines.

Once I get it bled and operable, I agree with you on the biannual flush schedule.

Orlando 1

When I brought a 69 Cutlass with 45k miles out of hibernation I discovered the front line was corroded inside but still moved fluid. It would build pressure at the wheel after a few pumps and braking was uneven. I thought it was a lot of things before replacing the line.

You have a great sounding project. Post pics when you can and good luck on your progress.

oldcutlass

Bi-annual flush... That's a bit overkill. I used to do it whenever I did a brake job, but I don't put those kind of miles on it now. Now every 5-6 years I may flush some fluid through, when I inspect the brakes and wheel bearings..