The car was due for a DOT 5 conversion... we just upped the implementation date. So far, we found one leaky piston and one bulging brake hose... so the work has already been justified. Plus, the front calipers had never been touched since I bought the car in 1986.... they were full of sludge. It was due for an overhaul. As of now, all four calipers have been disassembled, sandblasted, stainless steel sleeves polished, castings primed, painted, and left to dry. Jenny also honed the original 1970 Master Cylinder, and it has also been painted... ready to receive its new Bendix internals. Jenny has also been doing double duty by starting to assemble a a 327 for my C2 (checking piston ring end gaps). Busy busy.

So far, we're on schedule for testing early next week.

Ingar... that drawing of the valve is not what the (my) GM original has on it. The switch functions quite differently. There is no plunger, or no moving parts in my warning light switch. You will see when we post the pics sometime next week. I am familiar with the switch in the drawing... it is quite common, but is not as the original on a 70 Vette.




[Modified by Tom454, 11:21 PM 6/27/2002]

 

Continuation of above post regarding a real-world "test"....

Jenny & I finished installing all of the parts today.... that's the good news.
The bad news is that "old bessy" (original master cylinder) will not hold pressure for more than a few minutes... pumps up hard, but then loses it.

We isolated the M/C by putting plugs in the output ports. Then we used the built in bleeders to "bench bleed" the M/C.... several times.

It gets hard as a rock, but then quickly bleeds off.
We used a Bendix kit that included a new primary piston & seals (rear brakes), but only new seals (3) for the secondary piston (front brakes).
My guess is that the M/C has been honed too many times in its lifetime and the seals will not take up the slack. There is no pitting on the walls. So.... we are in search of a replacement.

The "proportioning valve" is now perfectly clean inside & out and it seems to be working:

It is impossible to do a "two man" bleed with the valve installed (on this particular 1970 Corvette). The instant the pedal is pressed with an open caliper bleeder.... the valve isolates the "leak" and plugs off that side of the system. It takes about 30 hard pumps of the brake pedal to get the valve re-centered after the bleeder is closed.

So.... apparently, my valve was not functioning properly because I have done the 2 man bleeder routine several times in the past on this car, and it worked. Not this time. We couldn't get any fluid flow to the open caliper bleeder with Jenny pressing the pedal and me manning the caliper.

It didn't matter how gentle/slow the pedal was pressed.... the valve responded instantly.... as it is supposed to.

Gravity bleeding was used to finish up.

More to come as it unfolds.... I just closed the doors and it's 3:20 am.... yawn.

 

Tom454,

THIS IS GREAT! You've got us all thinking now.

When you did the tests with the pressure differential system, did you have the switch screwed in? The switch should have a spring loaded post that sits in the groove in the "dog bone". It should not allow the dog bone to move if there is a light pressure difference, allowing you to gently bleed the brakes. If the dog bone moves....it pushes the plunger up into the switch, grounding the contact and making the light go on. Sometimes there is a step on the dog bone which stops it from going back easily once activated unless the switch is unscrewed to release the dog bone.

You said previously...

"The switch contact where the wire connects does not move... it remains stationary. The center post is isolated (insulated) off ground, and as the internal hydraulic parts slide off to one side during a system failure, the "dog-bone" touches the switch post and via the all brass internal parts, goes (connects) to ground. There is no provision for "proportioning" of fluid pressure whatsoever in a 1970 OEM brass "proportioning" valve. Not that I can discern"

This made me think that your switch plunger might be stuck.

Can you check if your switch plunger moves freely in and out of the switch. (The plunger is opposite the post where the wire is connected) You'll have to remove it from the differential pressure assy, but you should not lose any fluid, the switch plunger should be in the dry center of the assy.

I'll probably check mine now....it's probably gunked up and seized like yours was.

Many thanks for yor efforts!
Joe

'69 427L36 M21 3.08 White Coupe

 

Joe...

Question:
"When you did the tests with the pressure differential system, did you have the switch screwed in?"

Ans: Yes... the system is exactly as GM built it 32 years ago, except for stainless sleeved calipers, and stainless pistons from SSBC.

One other interesting thing... I did not install the front sleeved calipers... and never worked on them. When we took them apart, they were missing the piston springs. I did not know this, and have been driving the car for 16 years..... no problems. The front rotors are original (riveted), never been turned. So... we decided to leave the springs out of the rear calipers as well just to see what happens.

Question:
"The switch should have a spring loaded post that sits in the groove in the "dog bone". It should not allow the dog bone to move if there is a light pressure difference, allowing you to gently bleed the brakes. If the dog bone moves....it pushes the plunger up into the switch, grounding the contact and making the light go on. Sometimes there is a step on the dog bone which stops it from going back easily once activated unless the switch is unscrewed to release the dog bone. "

Ans: This is the point I was trying to make in one of my above responses... the OEM switch in a 1970 Corvette is not designed as you describe. The center post is stationary, and does not move whatsoever....it was designed that way. The switch you describe is a "text book" switch, which I have seen and worked on on many vehicles....but it is not OEM for a 1970 Vette. The center post of a 70 Vette OEM switch just provides a contact point for the circuit. The "dog bone" slides to one side when a pressure differential occurs, and the center post then contacts one end of the dog bone. The current path is throuigh the dog bone, to the o-ring piston, through the spring (identical spring on both sides), through the inverted flare seat insert, to the brass valve casting, to the steel mounting tab, to ground. All of the OEM valve parts are brass, except for the springs, internal switch contact point, and the mounting tab.

I have pictures to post when we finish up. Jenny and I are going to do that together. She likes that stuff. Then you will see how the valve is engineered.

If you really need to see... send me an email and I'll send you a few now.
I believe the 1969 valve is different than the 70.... perhaps as you describe.

Question:
"This made me think that your switch plunger might be stuck. "

Ans... there is no "plunger".... the "switch" is not a switch... it is simply a contact point.

Question:
Can you check if your switch plunger moves freely in and out of the switch. (The plunger is opposite the post where the wire is connected) You'll have to remove it from the differential pressure assy, but you should not lose any fluid, the switch plunger should be in the dry center of the assy.

Ans: The entire brake system was removed and completely disassembled... including the prop valve. The brass inverted flare seats were removed, and the valve internals were slid out and cleaned. The switch contact point was coated with a hard sludge, not able to make electrical contact in the event of a system failure... we have pics of that as well.
There is no mistake here... there is no "plunger" in this OEM 1970 switch.... apparently, unlike other year Corvette proportioning valves. The switch has the correct patent/part numbers stamped on the mounting tab and has never been removed or replaced.

Let me know if you want a few pics.
I'm uploading them from the camera when I finish this reply.

Tom




[Modified by Tom454, 9:29 AM 7/2/2002]

 

Dear Tom454,

It's late in Melbourne, about 1am.

I've just removed my "switch" and it is EXACTLY as you describe! No moving parts, just a contact point. So it's slightly different to the differential pressure switches I'm used to. Just forget all of my previous dumb comments about the switch.

Nevertheless, this is still a fantastic bit of research you're doing and I think a lot of us are following it with interest and learning. I think the little pistons might be stuck in mine, as I've not had problems bleeding my brakes, although I'm not entirely happy with pedal feel and travel. Might have something to do with it.

Many thanks for your patience with dumb posts like mine.

Joe

'69 427L36 M21 3.08 White Coupe

 

Good info in this thread. Just a note: The combination valve used on my '76 has no pin on the end of it. Others may be similar.

 

The 1970 valve also does not have a pin for bleeding purposes. No provision has been made for resetting the valve either. When the system is closed back up (repaired) , the M/C properly bench bled, and the functioning circuit (front or rear, non-bypassed) bled, the valve will return to center position by repeated hard pumping of the brake pedal. The valve must also be in perfect working order (no sludge), or it will stick and not return to center.

Mine had sludge in it, but the o-rings were fine so we re-used them.

Also- there appears to be a machined-in step inside the valve at each end to govern the piston movement.... don't make the mistake of reaming it out, thinking it's dirt.