IMO...the ONLY time you really NEED a porportioning valve is with drumrear/disc frontt brakes....the rear brakes are what's known as 'self-actuating' and they require less line pressure to activate and keep activated....
since the vette is 4 wheel disc, with small pistons as noted allready in the rear....the line pressure issue was mostly all taken care of in the initial design...
so the only possible reason they ever went to a 'real proportioning' valve might have been something to do with computer engines/carbs...and or trany changed with lockup converters.....meaning rear wheel drag was increased to further lessining of the rear brake pressures was necessary.....just guessing, but seems logical....dont forget they went aluminum suspension back there too in '80.......

about those adjustable line pressure valves....Dodge/Mopar has had those out for decades now...had one in a Pontiac I had some time ago....
it can NOT increase line pressure downstream....just LOWER it....
so it's not an aide for increased rear braking action....

I DO think and have for some time now, that increased diameter bore in the master cylinder is the way to go to get less pedal travel...
and that book suggestion of a 74-75 truck master cylinder is most helpful....
I wonder if that 20% increase in piston surface area will help all that much...it might....

at any rate, I"m going under the hood and see if I have which proportioning valve/or switch assy in place now....the '72 book on page 5-35 top of the page shows the vette blcok pretty clear, as being differant than the one on the right....proportioning valve.....

keep in mind this book covers all cars in Chevvy lineup for '72....

 

Wouldn't a bigger bore in the MC increase pedal travel? I found this at http://www.carcraft.com/ under the Tech Articles - Braking Points.

"....Multiplying the area of the cylinder bore by its stroke gives the total displacement (just like finding an engine’s size). The greater the bore size, the less the pressure but the greater the capacity, assuming pedal advantage remains unchanged. Changing the M/C stroke increases capacity but has no effect on pressure, again assuming the pedal advantage remains unchanged. If, for reasons of increased capacity, a large bore size is required that causes an excessive pressure drop, the pedal advantage can be increased, physical underdash clearance permitting. ..."

Or am I reading it wrong?

:cheers:

 

When you apply the brakes you have to move a certain volume of fluid. The volume moved is controlled by the bore plus a certain stroke. If you increase the bore less strock is required but the line pressure also goes down so while the required stroke from the pedal is decreased so is the braking pressure.
A manual master cylinder has a smaller bore plus increased leverage in the linkage under the dash to up the line pressure but the required travel is greater. With power brakes they up the bore size, decrease the leverage under the dash and add a brake booster. Pedal travel is less and hopefully so is the pressure required by the foot.

 

Tom454, doesn't your test now show that it's "safer" not having valve/switch? At least for that year?

 

Sure seems that way based on what I experienced...
After I removed, disassembled, cleaned and re-installed my valve, it worked as designed... and I had a difficult time bleeding the brakes, as I expected. The slightest amount of pedal pressure caused the pistons to slide off to one side. I had to bleed both one front & one rear caliper simultaneously to keep the pressure differential minimized... otherwise, the switch activated (slid) and the line being bled was blocked off by the switch.

Since I didn't try it, I can only guess that the brakes would be safer with two completely separate front & rear circuits... as long as the bleed between master cylinder chambers is minimized. The chambers are not 100% independent... there is leakage/flow between them.

 

one of my favorite long drawn out topics,,,,,,,c3 vette brakes :D

 

Was that enthusiasm? Cool. Great contribution to the forum. 4 stars for you.

As far as I know. no one else has taken their personal time as Jenny & I have to investigate these valves to the level we have done. So... you're welcome.

Now it's your turn to make a significant contributiuon to the rest of us kind people here on the forum.

Tom

Oh... sorry... I didn't see the :D
You were just messin with us. No problem.



[Modified by Tom454, 8:28 AM 12/3/2002]

 

I replaced my 1960 chassis with c-4 'chassis concepts' and it has porportioning valve for rear brakes. has anyone adjusted one of these for max braking? how did you do it? can't seem to get response from builder.

 

I think this might be the record for oldest thread resurrection. Well done!

 

I agree. This could be a record, responding to a post from 19 years ago.

Please ask your question in a new thread.