I have a '70SS (early production) w/ power brakes that I am trying to troubleshoot the rear discs.
This is actually a two part question, well maybe more.

There is this little black box you route lines to (car is not near me so I can't give a better description) that you bolt to the power booster & master cylinder. What does it do, and why do some model years not have it? Do I need it?
Second, the brass(I think) combination valve. That just meters fluid right? There is no residual pressure valve hidden in that right?

this rear brake thing is killing me! Then I get to change out the trans before I can even drive the car, and my weekends are booked for the forseeable future..

matt

Matt
I'm no expert but I am pretty sure the black box you are refering to IS the proportioning valve on a 70. It's bracket is held in place by one of the master cylinder bolts? In 71&72 GM used the brass type that bolts to the frame. It sounds like you have both. I don't know the history or if the car ever did work right, but I would start by looking into that. I checked a couple catalogs and they show the cast iron type that bolts up at the M/C as the correct one. It looks like you could use the flat brass type if you are not worried about being correct but not both. Let me know what you find.

Dan

http://community.webshots.com/photo/289630456/311749281QCtOzM

Dan,


Thanks for the info. I will be near the car tonight, but its raining/yucky here so I a not sure how good the shots I can take will be.
But yeah-- I do have that box, except mine is painted black. I also have what appears to be some sort of combination valve on the frame tucked in. I will give it a good one over and a report later tonight or tomorrow morning.

I really appreciate the help!

matt

Ok.. I was dumb and left the camera somewhere, but the pictures aren't very good anyway. But, my cast iron 'box' that is very similar to the one pictured in the link you put up, except the inlet/outlet ports are in slightly different spots. So it goes front brake master cylinder to cast iron box to brass valve on the frame, at which point it appears to split for both front brakes.
The rear brake line also goes into this brass box, and then just exits and goes down the frame rail. I have my adj. prop valve mounted after it.
Is the rear brake line going through that just a routing mechanism or does it do something to the fluid?
I am just trying to get ideas to hammer this all out.

thanks guys

matt

Matt
I don't feel comfortable telling you to get rid of the little cast iron one and just use the brass one... but if it were my car that's what I would do. I hope one of the experts will jump in with some insight. I am pretty sure you have two differnt styles of prop valves piped in series. That will never work right.

http://img146.exs.cx/img146/2374/p31100444fb.jpg (http://www.imageshack.us/)

this is the prop valve for the brass block

http://img146.exs.cx/img146/7479/p31100421zo.jpg (http://www.imageshack.us/)

this is a junction block from a 67
but it resembles 68-72
for drum brakes or disc/drum with prop valve pictured above

http://img146.exs.cx/img146/5186/p31100452ha.jpg (http://www.imageshack.us/)

and this is the prop valve that is used on disc/drum without the prop valve in first picture

as for front and rear disc brakes
are you sure you have the correct mc for that setup
I'm no expert with front and rear disc brakes
so if anything is useful to you in above posts I hope it will help you maybe
if not then sorry I am unable to help then

Tom/Dan
Thanks for all the help. I should be putting in the new master cylinder next friday afternoon. I got a short pushrod for it. I will then jack the car up & check for kinks and whatnot. If that doesn't work maybe I'll try a temporary test of black box removal.. I hate bleeding brakes...

matt

Tom,


On that third picture--- where it says "and this is the prop valve that is used on disc/drum without the prop valve in first picture" does that have a port for the rear brake lines?

My brass block has two coming in, one front line (from the little box near the MC), then one that goes in (from the part of the MC that supplies the rear brake pressure) and then out the side to the rear. Its real hard to see in there so that is the best description I can give. the thing next to the master cylinder on my car looks to be just about the same as the one in the linked photo you put up earlier, except the connections for the tubes are in slightly different places. Mine has a rubber end over one part where yours connects in, mine connects in in a different spot, but looks almost exactly the same (and fits the same) otherwise.

matt

the third picture

to the right pointing right and poining down are two different holes for the front brake lines
to the left is a big nut looking thing that has a single hole that feeds the rear brake line
it doesn't need two because you have the tee at the rear axle housing on the brake hose

if you have the first prop valve (in first picture) then you don't want the third pictured block

and the second pictured block has holes on both ends
one will match the front brake line and the other will be a bigger hole
the left side of that one would be for the front brakes and the right side is for the rear

Thanks tom. I have both the first and third. I'll be removing the 1st (easier) this weekend and bleeding the system. Will give it a whirl.

thanks

matt

you're welcome

I just hope this helps you out
but I would imagine this stuff would only affect the front brakes
give it a try though

Tom its my misguided hope that with the front brakes being double restricted to do less work, its the rear brakes doing the stopping and the front brakes only when I really wail on it. I'm not sure if it even makes sense to me but I'm hoping the new MC for 4 wheel discs, removal of that box, rebleed the system (don't suppose anyone has developed a quick way of doing this..) and then playing with the prop valve in the rear and see if it fixes it.
The guy at the baer tech line (not incredibly helpful...) said he though it was a kinked line or bent/hurt hose or frozen caliper. Frozen caliper maybe, but I gave the lines a quick one over yesterday, will do it full and well when its sunny ouy and I can see, but since the problem is with *both* rear discs it limits the # of places there could be a 'kink'

will update next week if the weather is good.

matt