I have the original master cylinder for my 70 in a box. I recently used it for testing another forum members brake system.... although it has a few pits in it, it still works good.

This master cylinder has the bleeders in the side of it.

If I were to install & bleed this M/C (and the rest of the system), then remove one of the master cylinder output lines and one bleeder, and connect a properly constructed steel tube to allow fluid to flow from the output port directly back into the master cylinder through the bleeder port (simulating a leak/failure).... and then drove the car....would this be a good test of all the ideas we are kicking around (assuming my combo switch is in perfect working order) ??

What would y'all predict?

I am willing to perform this test if you guys think it would serve a purpose.

Don't worry about the driving part being "naughty" or "dangerous".... I never had apron strings tied to my butt, and never will.

 

I'll be the first one to call :bs on that one. I have had failures on both systems and when that happens, I had virtually no brakes during either incident! my brake pedal went to the floor.

 

Great idea, that should settle some of the debate. I predict the other side of the system will take over braking duties :smash:

 

If you just jacked your car up and removed one wheel regardless of which and opened the bleeder screw and then jumped in the car and hit the brakes really hard a few times would your peddle go to the floor, would you feel safe driving the car?
While bleeding my rear brakes with my son doing the honors of opening and closing the bleeder screws I felt the peddle was still firm with the screw open and the fluid poring out the screw. I would not have worried at all taking the car out on the road with just the front brakes working.
My wife came home twice with a older Bonneville with blown rear wheel cylinders and both times she make it home OK but the peddle was almost to the carpet before it would stop. Very scary.
Forget the safety issue and just concentrate on the increased braking power from a performance point of view. I definitely noticed a big improvement in rear brake performance after removing this proportioning/light brass block. I would definitely do it agian to any fair weather vehicle I get in the future. The before and after testing convinced me this is a step towards better more aggressive braking.
It is very nice of you to offer to do the testing but is it worth all the effort?
You bleed lots of brakes in the past and in the future. Remember how the peddle felt when someone openned the bleeder screw? I pressure bleed now so the memory is fading.
Have a nice day.

 

"It is very nice of you to offer to do the testing but is it worth all the effort?"

Ans- I think it is... might answer some of our questions. Brake systems are a piece of cake for me.

"You bleed lots of brakes in the past and in the future. "

Ans- Affirmative on that.... I have been classified as a "professional" mechanic for over 30 years. Not that it means anything.... but I have done a LOT of brake jobs. Remember... when you do brake jobs for other people... you are responsible (and liable) for their lives.

"Remember how the peddle felt when someone openned the bleeder screw? "
Ans- Yes... I bled Jennys brakes yesterday just for fun.

I just want to know if you guys think it will be a valid "test" BEFORE I go through the trouble. I wouldn't want to do the work and then have some dude come on here and say the test is dumb and invalid.

Sooo.... what do y'all think?

 

Also you would feel the difference when breaking after one side of the system failed which would tell you to deal with it immediately. If both sides failed you would be screwed with or without the block.

 

Tom
i think EVERYBODY should check this out next time they bleed the brakes.
all anyone has to do is push on some fish tank hose, route it away from the tire, tie it up so it wont get tangeled. then open the bleeder 1 turn and go down the driveway 10 mph and slam on the brakes. (make sure there is plenty of room to coast to a stop :rolleyes: and prepare to yank the hopeless e-brake :sleep: last chance to avoid collision: shift into R :cry

 

Tom, if you are willing to take the time to do this test, I think the results may be very interesting.
For myself, I would like to know the answer.
If the answer is NO BRAKES, then Norval makes a great case to remove the block. (For safety reasons if nothing else)
If you still have brakes, but after the second pump, or something, it would be nice to know.
In an emergency situation, if the knowledge of how it reacts to a loss of fluid is in the back of your mind, it may save your butt someday.

 

Okay...

I'll give it a shot.

Should be done before the end of next week.

I believe I have an assistant (witness?) lined up as well.
Should be fun.

 

Tom, doesn't the bleeder thread into the same high pressure area that the output line connects to? I would think that a line between both ports would act the same as if you plugged both port.

Am I missing something? (again...:lol:)

-Joe

 

Yup Joe... I think you are correct. Brain Fart on my part.

So instead, I'll take a junky old spare cover and drill holes in it.

One way or the other... I'll test the valves function under stress.
I'll take a closer look at the old clunker tomorrow.

Thanks for the heads up Joe.

 

Hey, even a broken clock is right once a day :lol: Glad I could help.

-Joe


[Modified by Flareside, 7:24 AM 6/21/2002]

 

Joe, Do you only have 12 hours in your day? Craig

 

As pointed out by Joe, the master cylinder is designed so than one circuit will maintain pressure if the other developes a leak.
However, since it will take some fluid to move the piston in the differential valve, the pedal will drop on the first pump after you open one circuit.
May this is what Norwal experienced?

 

We'll all know in a few days.... when do the "test".

The point has already beem made that when you are bleeding the brakes on a C3 with a dual master cylinder (as opposed to a C2 with a single reservoir) , and you crack only one bleeder, the pedal goes to the floor.

Lets see what happens when one output port is wide open, simulating a ruptured brake line as occurs with rusty lines in the snow/salt belt. (which I have personally experienced). :)

 

The project has begun... while I disconnected the brake lines, Jenny removed the Master Cylinder and all 4 calipers. Caliper Kits were purchased at AutoZone, while a M/C rebuild kit (Bendix) was ordered & received from Advance Auto. I already have several quarts of silicone fluid (DOT 5) in stock.

The distribution switch did not come out without a struggle... the bottom inverted flare nut & tube twisted off due to rust, so I have to repair the damaged line..... and this will add a little time to the project. I expect to just splice in a piece for the test, but will order new stainless line(s).

I expect to have the brakes re-installed and the test completed by Monday, because I intend to drive the car to the Stars & Stripes event being held at Performance Chevrolet in Chapel Hill on Monday ( http://www.grammonline.com/starsandstripes ).

The switch ("proportioning valve" ) has been disassembled, sandblasted and refinished.

It did have some sludge in it, and the electrical contact portion of the switch was obviously malfunctioning..... it had a thick, non-conductive coating of sludge on the exact spot where the circuit would normally complete (inside the switch) to turn the light on in the event of a hydraulic pressure loss.
I know several people have recently asked if this switch can be rebuilt...
The answer is yes for a 1970 OEM brass switch... it has two O-rings inside, and two o-rings to seal the switch post insert. To disassemble it, we tapped the inverted flare seats in both ends with a 6-32 tap, mounted the switch in a bench vise, and used a machine screw to (very carefully) pull the seats out of the ends of the switch. Once the seats are out, and the switch contact post is removed, the internal parts can be pushed out of the switch. If the switch post is not first removed, you will destroy the valve trying to get the internal parts out. Here are the parts....end-to-end...

inverted flare seat
spring
piston with o-ring
"dog-bone" switch contact slider
piston with 0-ring
spring
inverted flare seat

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.

The hydraulic portion of the switch appears to be working properly.... no pits or scoring is evident.

We have pictures and will post when the job & test are complete.

FYI- The OEM Chassis Service Manual has a good description of how the dual master cylinder is supposed to save our lives.

If I can find the time, I will post that as well.
Tom

 

I appreciate you taking the time to do some research to benefit all of our knowledge bases. I am going to do some extensive dyno testing on my current engine and 383. I will post a list of things I plan to change and dyno after each just to see what it does. I want to see hard data on what changes I make have the most/least affect on my power ratings and which ones are worth the trouble and expense.

 

Tom and Jenny. You guys are really getting into this. Removing the master cylinder and all 4 calipers plus rebuilding the proportioning valve. Hope this is needed and not just done to find out what works and what is needed. I thought you would just remove one or open a bleeder valve to simulate a ruptured brake line and take it for a drive. If you get the chance and you ARE going to alot of work for us try the system with and without the proportioning valve and see if the back brakes work that much better.
If you were local i would gladly send you the replacement silicone. I have it by the drum full.
Good luck in testing and thank you for the effort you and Jenny are making for the rest of us.

 

Here is a split drawing of the master cylinder. If one circuit is open one of the springs must be compressed before pressure rises in the other circuit. Which means that the pedal will drop lower than normal.



This is the combination/differential valve. Can't see any interaction between the two circuits so I guess this is the combination type (pre 75).
The mechanism at the right side I assume is to center the piston after the switch has been activated.

Both images from the Haynes manual. (no split drawings in my 1980 shop manual)

Anyone who has a split drawing of the later differential type?

 

Yeah, thanks a lot you guys - this will be good information. And thanks for starting the thread Norval.