Master cylinder to proportioning valve plumbing

I was finally able to get the hydraulic unit for the hydroboost in on Sunday. Lost the clevis pin, but that is another story :cussing:

Having read a paper recently that suggested "dry fitting" the master cylinder before priming, I put it in only to find that the old one was plumbed in reverse to the new one. Let me try to explain clearly:

Old MC: Large MC plug (5/8 X 20 I believe) in the back going to the rear wheels. Top port on proportioning valve. Small plug and line to the fronts ( 1/2 X 18?). Bottom input port on proportioning valve.

New MC: Large MC plug (5/8 X 20 I believe) in the FRONT. Small plug ( 1/2 X 18?) in the back.

The car is a 73 and hydroboost requires a later MC or a spacer bar in the booster shaft. My old MC was bad anyways so had to replace it and went with the newer model.

My question is, which is right? Should not the larger fitting and line go to the front brakes? My brake booster was certainly not original so there is no reason to believe that the MC was. The lines look original but could certainly have been switched around.

If you take the m/cyl apart, I think you find a stiff spring between the front and rear pistons.....

so the rear section applies the rear brakes first, and some of the power to the front....that is the way I have my aluminum aftermarket m/cyl plumbed....

that brass pressure bias switch there, with all the lines in/out of it....is gone from my car, I used a splitter in the front line to the front wheels, and then the rear line goes directly into the m/cyl.....

it's the way Norval recommended and did for his car, and he is correct, that old bias switch flipped one way or another to signal a huge pressure differential and light the dash bulb....problem is....that switch itself tends to leak...it has a fiber washer and a O ring in it, as I recall.....other than that, there ain't nothing there.....

:gurney::beer:

So the pressure bias (rear reservoir) should be to the rear wheels? How about the volume bias? Are the front caliper pistons not larger than the rear?

You're not the first one to suggest ditching the proportioning valve. For now, I plan to leave it but if it shows any signs of leaking, I don't intend to replace it.

73 Mike said:

So the pressure bias (rear reservoir) should be to the rear wheels? How about the volume bias? Are the front caliper pistons not larger than the rear?

You're not the first one to suggest ditching the proportioning valve. For now, I plan to leave it but if it shows any signs of leaking, I don't intend to replace it.

Click to expand...

I have never messed with that valve/switch on later sharks, they SAY it's a proportioning valve....but MY brass dist block there with the brown plug in wire was just a brake pressure bias switch....which could unscrew from the brass block and see nothing there but a simple slide switch unbalance the line pressures either way by enough that switch ground the brown wire and lights the dash bulb....

I took it out, and it the car works fine.....

:chinese:

OK, here's what I've been able to learn today. First of all, the plugs are 1/2 X 20 and 9/16 X 18. So much for my memory.

Second, the large plug and line is supposed to be in back as per my original MC. The one I got with my hydroboost is wrong.

The last thing that I have yet to confirm is that the larger caliper pistons are in back. I only have a rear casting so nothing to compare it to.

The best solution would be to go out and get a 76 to 82 MC. The next best, and the one I'm going to do, is use the brand new 73 MC hat I have and put a spacer between the pushrod and the back of the MC recess.

The caliper pistons in the FRONT are larger than the rear.

big2bird said:

The caliper pistons in the FRONT are larger than the rear.

Click to expand...

Wouldn't it then follow that the larger diameter brake line feed the fronts?:surrender:

:suicide:

Thought I had this figured out.

73 Mike said:

big2bird said:

The caliper pistons in the FRONT are larger than the rear.

Click to expand...

Wouldn't it then follow that the larger diameter brake line feed the fronts?:surrender:

:suicide:

Thought I had this figured out.

Click to expand...

NO, it matters not, just the amount of fluid has to go through the smaller line FASTER to still displace the same fluid amount.....maybe there is some amount of action there in a panic stop that makes the front brakes apply a micro second later than the rears, han'dt thought of THAT, humm.....interesting.....

but one thing I have found that seems stupidly frustrating about GM brakes over the years is their constant use of odd sized nuts and line diameters so when buying steel brake lines, it's a constant battle with haveing to reuse the old nutz or buy more parts to get the nutz, then cut and reflair the lines to match up...talk about a PIA.....and for NO apparent reason....

it is interesting though about that spring and the smaller front lines delaying the panic action by some micro seconds on front application....rears first is always they way I learned it.....

:fishing:

mrvette said:

If you take the m/cyl apart, I think you find a stiff spring between the front and rear pistons...

Click to expand...

73 Mike said:

OK, here's what I've been able to learn today. First of all, the plugs are 1/2 X 20 and 9/16 X 18....
The best solution would be to go out and get a 76 to 82 MC. The next best, and the one I'm going to do, is use the brand new 73 MC hat I have and put a spacer between the pushrod and the back of the MC recess.

Click to expand...

Yes, the 76-82 plugs are 1/2 X 20 and 9/16 X 18 with the largest one at the rear. When I first got my 82, the MC that was on there had a larger rear reservoir that I believe was for a GM vehicle with a vacuum booster but rear drum brakes. The car seemed to stop ok though.

Does your MC have different size reservoirs? Who'd you get the hydro unit and MC combo from? Tell them they sent you the wrong MC...

My '72 came with a m/cyl that had a hole in the back piston in which the adjustable pin from the booster went in by about 3/4-1" worth, later sharks came with the much more common m/cyl look in that rear piston which was nothing but a small centering divot for the booster pin to rest on/in.....
why the change, I dunno, but obviously there is no compatability on that score....AFAIK the two had identical internal piston diameters.....1".....

in the middle of battling these stupid brakes for every damn fault that was imaginable and they all well documented, I changed to a slightly differant appearing but other wise identical TRUCK m/cyl.....the diameter was 1.125 or so for the pistons, with the line sizes reversed on the connections....I didn't think it would make much if any differance, and just reversed the lines so they bolted up....car seemed to stop better mainly due to larger diameter, but would grab the brakes in front too soon....they would lock up first, found out in the rain one day...the pedal was much more firm, and the travel a bit less, so overall it was an improvement, but as time wore on, and the system wear went on, the pedal somehow got softer and softer, and never did find any air in the lines....

so then hearing of this Hydro Boost conversion and the rave review on CF about them, I overpaid off ebay for a unit and it came with the stock hoses from the tear out of unknown vehicle....it was a truck is all I remember....
so upon installing, my car stopped on a dime even before the air was out of all the fluids, when it all settled down and quit moaning and groaning over a couple of trips, the brakes became even more sensitive, but it was a pure pleasure to tap the pedal and actually STOP the damn car instead of wish and pray every time as the pedal went to the floor.....damn nearly....

so I ordered a aluminum m/cyl from Pirate Jack racing....China I"m sure, but it been fine so far....so this time the rear brakes went directly to the rear port, and the front got a brass splitter block, and they went to the front port on the m/cyl.....the whole braks bias thing went away, car still stops on a dime, pedal is very firm, that damn squishey crappy 1/2 assed feel that all vacuum boosters have in them is GONE,

never been a car with a vac booster I couldn't plant the pedal to the floor with enough force, that to me is shitty design and I don't care who says what,

upon tearing apart my old power booster, I could find absolutely NO internal defects, pulling the suck valve out of it made a nice huge air inrush so it was holding vac nicely....the rods all seemed sealed, diaphrams fine, seals fine, no internal rust, input shaft, output shaft seals seemed fine, but I noticed one thing.....another damn spring inbetween the input shaft and output shaft....it was a tough bastard I will say that, harder than a valve spring even....but still that give in there, which probably explains that ability to push the pedal to the floor at will....

something I can not do with the HB unit....

The HB is the last of the brake wars, having O ringed the calipers and ditched the springs many years earlier....car came with stainless calipers that are still on there....

I found in my '72 that the 'proportioning valve' is nothing but a line pressure differential switch, and as such would light the dash bulb when one line went dead, front or rear....which was a good thing to light that bulb as the pedal went to the floor due to piss poor lip seal design and them silly springs which accentuated the 'sudden failure syndrome'......to boot that brass switch seemed to have a leak in the switch, so hell with it and the system is fine....

:harhar::harhar::harhar::smash:

only took my about11+ years to finally win the great brake war, but I won, so eff'em.....

:2nd: