Combination valve mod

[hgerhardt]

Hi all, first post here...
I have a '66 GTO that I'm replacing the brakes from 70's F-body front and Seville big calipers in the rear to C5 front and LS1 rear. Piston area calculations show that the C5 caliper has roughly half the area as the iron single-piston calipers, so that tells me I'll need roughly twice the pressure. I'm going to try my B-body powermaster master cylinder with the pedal pushrod raised to about midway between the stock power and manual brakes position on the pedal arm and see how that works. I'm also using the '79 Seville combination valve for now, as it's already installed.

Now the big question is, with the roughly double system pressure I'll need for this swap, how has anyone modified this valve to increase pressure to the rear? This is the style of valve that has a plug on the bottom where the spring and piston go. Spring specs, anyone? Don't have any fluid in the system yet, as I'd like to get the spring at least in the ballpark first.

Thanks,
Heinrich Gerhardt
'66 GTO, EFI 428, 200-4R

[oestek]

Hey Heinrich,

Welcome to the board! I don't know if you remember me, but I was a Producer on Hot Rod TV when we featured your GTO. For those who don't know Heinrich, his blue '66 was a Hot Rod cover car back around 2000, and it is jammed with killer tricks. Lots of Heinrich--made carbon parts and custom machining. Great car. Is this the same one you're working on?

- Kevin Oeste

[hgerhardt]

Hi Kevin,
I vaguely remember you... that was a few years ago! Yes, this is the same car. A woman ran a stop sign about 7 years ago and wiped out the entire RH side of my car with her Volvo and I'm just now getting the thing back together again. Her insurance has paid for it all so far, but how do you put a price on the aggravation?

Funny story about the TV shoot... if you remember, I didn't install my M&H's for the shoot and consequently only ran a 12.-something with major wheelspin. Well, on the way home, in the fast lane on the 405, I came up to a left-lane slowpoke, put my foot into it to get around him, and just as the trans downshifted, I heard a small "bang" and suddenly I had neutral. Coasted over 5 lanes and managed to get the car to the right shoulder without getting run over. Sheared off the intermediate shaft in the 200-4R. If I had installed the slicks at the TV shoot, it would have broken on the track, on TV, and that would have been embarrassing!

[oestek]

That would have been bad, but not the first time it happened! Sorry to hear about the wreck... knowing your abilities, it will come out great. Do you have any pics you can post to show the forum?

[Apogee]

Have you considered gutting the factory prop valve and running an adjustable version instead? In theory, you could do that, test and tune it the way you like, and then reverse engineer the adjustable prop valve and then recreate those results with the OE style valve. Probably sounds easier to do than it is, but that's probably the direction I would go until I saw a better option.

Tobin
KORE3

[hgerhardt]

Have you considered gutting the factory prop valve and running an adjustable version instead?

Yeah, I thought about that too.

Seeing how so many people have already installed C5 brakes (in fact, that's the main reason Tobin's company exists, I see), it seemed logical to me that someone had already done it and I could skip the R&D. I guess I'll take it apart and see what size the piston is and how stiff the spring is now and go from there.

So here are two ideas for you, Tobin: make parking brake cables for LS1 rears to fit into 64-72 A-bodies and a reasonably close proportioning/combination valve for C5 conversions.
Thanks,
Heinrich

[Apogee]

Most run the aftermarket adjustable proportioning valves due to relatively low cost, ease of installation, and effectiveness in use. So many cars are in constant state of flux that it doesn't make sense to design adjustability out of the system when it's possible to add it instead.

Thanks for the suggestions. We'll take them into consideration.

Tobin
KORE3

[HWYSTR]

Heinrich - I agree with Tobin, ditch it, and go with an adjustable. I've been in the same boat, and no combo valve I've found really fits the bill perfectly. I'm switching as soon as I feel motivated enough on that project!

As for the 2004r, after several in my car, I recently switched to a 4L80E, and will never go back!

I remember your car very well, sweet ride, and nice job! The layout from the mag is on the wall in my garage!


-Mike

[64Chevelle]

So here are two ideas for you, Tobin: make parking brake cables for LS1 rears to fit into 64-72 A-bodies and a reasonably close proportioning/combination valve for C5 conversions.
Thanks,
Heinrich

Excellent ideas!

[hgerhardt]

Heinrich - I agree with Tobin, ditch it, and go with an adjustable.

I suppose that's what I'll do. I have one from Wilwood that I bought at least 10 years ago. Hopefully it will have a high-enough max pressure.

As for the 2004r, after several in my car, I recently switched to a 4L80E, and will never go back!

I know I SHOULD do that... But I'm a cheap ba$tard and I don't want the extra weight. For now, it's no big deal to change out the 3rd clutch and band every so often. Besides, I just had Continental add 500 rpm stall speed to the converter... of course, that's going to add more stress to the trans... we'll see. I might be changing the trans sooner than I'd like.

I remember your car very well, sweet ride, and nice job! The layout from the mag is on the wall in my garage!
-Mike

Thanks! It's amazing how many people still remember the car. That was 10 years ago!

[HWYSTR]

Should be good on the prop valve, shoot, I used a Direct Connection one for years! (Actually had a Kelsey-Hayes sticker under the DC one!). They usually are 0-50%, if that makes any sense. And no chance for the internal rod hanging.

If you're good at building the 2004r, and you get them to live a while, and are ok with going through them every now and then, stick with it! Mine's a daily driver, can't afford too much down time, and had to send my trans out for rebuilds. Once I reached a point I was like ok, what WON'T I kill! I really like the fact that I can control shift firmness, shift points, etc, all relevant to throttle position. It's also nice when I let off the gas the converter unlocks. And it's what, like 65 lbs heavier than a TH400. Eh, I can live with that.

I'll have to take a snap-shot of that page on my wall! I look at it every day just about! Certainly has been an inspiration on a number of occassions! Never let it go!

.