Help solve my drum/disc incompatibility--what MC?

[GoldHawg]

Sorry for the repeat post--I thought I was able to modify title to get to my point but it stayed the same as my original, so here is same ? w/correct title.

I have a 71 Cutlass that I converted from stock 4 wheel drums to camaro 11 in disc brakes w/f-body spindle and MC many years ago, but I did not change the proportioning valve (not necessary). I later upgraded to the "poor man's disc" rear from MP Brakes (11 in drums, big improvement) and that really helped the brakes. So I then decide to upgrade the front single piston Camaro calipers to the Stainless Steel brakes "quick change" dual piston caliper. After that swap the brakes barely worked at all. Neither MP nor SS Brakes were helpful at resolving incompatability; many of the website discussions suggested piston area differences caused the problems. I got frustrated and just put the single piston calipers back on and brakes are fine.

Hoping I can get some more specific feedback on what I need to do to make the dual piston SS front calipers work on my Cutlass. What Master Cylinder do I need (currently have the mid-70s Camaro disc/drum MC)? Do I also need to change the drum proportioning valve? If so, can I just put a stock disc proportioning valve or must I get an adjustable one? Thanks for any specific feedback. I'm betting the tech advice here is probably better than I was able to get from SS or MP Brakes!

TIA.

[Roadbuster]

GoldHawg,
A couple of questions:
1. Do you have power brakes?
2. How big is the Master cylinder?
3. By proportioning valve that you did not change out I think you mean the distribution block? Two lines in from M/C and three out to brakes?


My thoughts:
A proportioning valve is not a bad idea with your system. You have a combination that is not factory big drum / aftermarket front disk combo) and will allow you to set the front/rear bias of the system.

The new front brakes most likely have a larger piston area. This could mean a M/C change. That combined with the larger rear brakes is a concern the current master is adequate for the new brakes.

If you have the integrated distribution block with delay valve you are set for a disk/drum system. If not consider getting a delay valve which is a disk/drum only part. You can get the delay valve as a separate part and install it in the line to the rear brakes (1968 and I think 69 disk/drum A bodies had this). This prevents the rear brakes from locking up before the fronts in a disk drum combo.

I am in the process of working out my own brake system - Baer fronts, factory drum rear. Hydroboost with Wilwood Aluminum M/C and prop valve. I decided to reuse the distribution/delay valve (71 or 72 A body style) that was in my car when I got it.

Hope this helps.

Jon

[GoldHawg]

I do have power brakes, although I have a smaller MP booster (9in) that I needed for a previous set of headers to clear (although now I could go back to original if necessary--headers are gone).

The MC is whatever was stock for mid 70s F-body front disc/rear drum.

I think you're right re distribution box. I do have one that was for a disc/drum car, but it was in a junkyard car w/brake lines cut--not sure I can reuse it.

I think I do need a new MC, but I don't want to change but once. If I am condemned to trial and error w/the MC, I will stick w/existing front single piston calipers. But since I paid $500 for powdercoated dual piston calipers, I'd sure like to use them.

thx

[Tito]

Which SSBC caliper upgrade did you get, the "Force10" one with twin 38mm pistons or the "Force10 HD" one with twin 43mm pistons? (Or just measure your piston sizes....)

The "stock" 2 15/16 calipers have 6.77 sq in of piston area
The 38mm SSBC calipers have 3.52 sq in of piston area (52%)
The 43mm SSBC calipes have 4.50 sq in of piston area (66%)

To keep the same pedal force to clamping force ratio, you'll need a master cyl with a similarly smaller bore (you most likely currently have a 1 1/8 bore)

SO, for the 38mm pistons, you'll need a master cyl with ~13/16" bore
and for the 43mm pistons, you'll need a master cyl with ~15/16" bore
The problem is going to be trying to find a master cyl with that small of a bore that still moves enough volume... "Cheap" guess if you have the 43mm pistons would be to use the 1" bore master from a 68-76 corvette with MANUAL brakes (it'll should still bolt up to your booster, etc.) This might still be too big of a bore though. Moving to a dual diaphragm 9" booster would likely help as well by adding more power assist.

You will almost definitely need a different (or an adjustable) proportioning valve in the rear line to keep the rear drums from locking up. You may also be able to change out to a smaller rear drum wheel cylinder to help get the proportioning a little more back in line...
Honestly, I've read a few comments "around" from folks who moved to the SSBC twin piston calipers and I can't recall hearing about anyone who was really all that happy with the results (similar to you -- couldn't get the fronts to lock). Maybe the stuff is just fine, but I just haven't seen many positive comments....

[yellow heap]

A manual master cylinder will have the long pushrod (deeper)hole for pushrod retention
A power master cylinder has a short hole.
a aftermarket or a factory alternative power Disc/disc is needed,probably a 1" or maybe a 15/16 bore.
I would dump the prop valve and plumb a aftermarket adj valve on the rear circuit
I think the 70 corvette was available with power disc/disc brakes,those master cylinders are not too spendy.
If you want to keep your brake light there are prop valves with light provisions.

[Tito]

A manual master cylinder will have the long pushrod (deeper)hole for pushrod retention
A power master cylinder has a short hole.

Actually, it depends on the year/make of the car. I know that the early 67-69 camaros, 68-72 chevelles and corvettes through sometime in the mid 70's used the "deep" hole master for both manual AND power brakes. As far as what is "right" for GoldHawg, it'll depend on what booster he's currently running...

I do have power brakes, although I have a smaller MP booster (9in) that I needed for a previous set of headers to clear (although now I could go back to original if necessary--headers are gone).

The MC is whatever was stock for mid 70s F-body front disc/rear drum.

Do you mean (literally) that the master cyl you're running is for a mid-70's camaro with power disc/drum? If so, then you probably have the "shallow" hole master/booster combo. In this case the 'vette master I mentioned won't work (unless you swap out the pushrod on your booster). BUT, you should be able to use the master cyl for a 70-74 camaro with MANUAL brakes (which has the shallow pushrod hole and a 1" bore)

[GoldHawg]

Yes I meant literally. I swapped over from 4 wheel power drum brakes to the negative camber tall gm spindle (using a 77 camaro spindle and calipers). I replaced my MC with a 77 power disc/drum MC from a Camaro.

My current plan is to replace the distribution box with a factory combination valve (which includes the proportioning valve) and try an LS1 MC per the other dualing post (sorry I have this on two posts). I picked up one reasonably cheap at the wrecking yard this weekend so I'll give it a try, and I'll plumb in the 10lb residual valve.

Any reason this approach is dumb as dirt?

[Tito]

Any reason this approach is dumb as dirt?

Well, the LS1 master cyl will have the outlet ports on the engine side and have metric fittings. I don't know if it's a quick takeup style or not (I don't think so, but I'm not sure) but if it is, then it might not fit in your current booster....

That said, if it bolts up to your booster and you're OK moving some plumbing around, it might be OK. 1" bore is still going to be a little large I think, but it might be workable.

If it were me and I was going through the trouble to re-plumb everything, I'd definitely put in an adjustable proportioning valve and i think I'd try to set it up so that I could use "some other" master cylinder (with a different bore) if the one I tired first didn't work. I'm not sure what other M/C's exist that are similar to the LS1 style. Figure if you try the LS1 and it's still too hard of a pedal, then you'll need to fins another master and re-plumb. If the LS1 does work, but the prop valve doesn't cut the rear pressure enough, then you'll never be able to keep the rear end from locking up...

[GoldHawg]

BUT, you should be able to use the master cyl for a 70-74 camaro with MANUAL brakes (which has the shallow pushrod hole and a 1" bore)

Well, I finally got around to reinstalling the SSBC calipers to see if I could make them work. I had prepared by purchasing a proportioning valve at the junkyard, along w/an LS1 F-body MC and a residual valve. So I get the car jacked up, and SURPRISE--there is a stock proportioning valve already on, not the much smaller distribution box. I hadn't really looked at it before, because I personnally swapped the stock drums off the car in 1990. I don't remember swapping the proportioning valve out, and am almost certain I didn't, because I would've had to make new double flare fittings and lines, and I didn't have that skill in 1990. I also seem to recall the instructions for the F-body brake install didn't require it. I have no idea how/when it got on there--shouldn't have been factory as it had drums when I bought, and I'd swear I didn't put it on in '90. In any case, given I already had a proportioning valve on I decided to try a slightly easier route, and just get the manual 70s Camaro MC recommended above. Only cost $20 at Pep-boys, and relatively quick to swap out and bleed brakes.

So, I buy the MC and bench bleed the MC well more than necessary, certain I got all air out, and put it on. then I bled the brakes and had no air in the system. But I cannot get any pedal feel--the brake pedal goes to the floor and does not grab. I'm certain I bled the brakes properly (and I tried again this morning very carefully just to be sure). Only thought I have is that this manual MC will not work somehow w/my power brake system.

when I had my original problem of the fronts not grabbing, I had plenty of pedal, it just wouldn't grab. This acts like I haven't even bled the brakes...

Thoughts?