Blazedog asked about proportioning valves and I don't want to hijack his question. Mine concerns the need for residual valves and the need for them in 4 wheel disk brakes. When building my car, when it came to doing the brakes I knew less than zero on doing it right. I had bought a few "how to build a hot rod books" through the years and referred to them for guidance. A couple of them out of five or six said residual valves needed to be installed for the front and rear lines. So I did that along with the proportioning valve. Well the brakes are not stellar by any means, but they get by but need rework. This is a Jag Series one suspension btw. One complaint by my cohort who stepped in to help me wire and finish getting it on the road has always said the valves should not be there, both the residual and the proportioning valve, so what say you guys? I'm currently waiting for kits to rebuild the front calipers. I really don't want to tare into it and replace those lines and actually can't because of health issues and if needed will have to go to a brake shop to get done. BTW the master cylinder is lower (under the floor) than the wheel cylinders which tells me the valves should stay because of bleed back. thanks for your advise.
Get a pressure tester and see what pressure you have, then go from there to figure out how to adjust pressure to make the brakes work like they should. It shouldn't be a guessing thing. https://leedbrakes.com/p-33765-leed...6P10_mL7CxZwF-7d7xwGplP6t9iapiGxoCAEcQAvD_BwE Sent from my SM-G973U using The H.A.M.B. mobile app
The residual valve does just as it's name implies, keeps a residual pressure in the line to have a slight pressure against the seals and keep the line full especially when the MC is lower than the calipers or wheel cylinders. If you ever had one of those old single piston under the floor master cylinders apart you remember the "check valve" in the front. That has a small spring inside it and in actuality is the "residual valve". The proportioning valve has nothing to do with the residual valves and vise versa. it might be better known as a front/rear balance valve because if you have an adjustable one you balance the front/rear braking bias with it. If you have locked up the brakes on a older long bed pickup with no load in the back you fully understand brake bias that isn't working right. If the back brakes lock up the back end comes around on you.
Larry, I'll have to agree with your friend. I get the need of residual valves if running drum brakes, the OEMs used them widely no matter where the master cylinder was mounted. But no residual valves are used OEM for discs that I'm aware of. Proportioning valves? I'm even less convinced these are needed. I've installed two under the advice of 'experts' and in both cases ended up running the valves wide-open after parking lot 'testing'. I'll send you a PM to discuss the more OT aspects of this...
Try driving the car for a few miles without using the brakes coast to a stop and check the brakes if they are hot remove the residual valves
Residual pressure valves are only needed when the master cylinder is lower than the brake cylinders. Two pounds for disc brakes and 10 pounds for drums. Proportioning valve is to adjust the pressure bias between front and rear brakes due to the difference in front/rear weight distribution during hard braking to keep the rear brakes from locking up way before the front. I would imagine some cars may not need much of an adjustment, but a little light car will.
We used Jag disks on a Model A coupe years ago, Just used every thing off the Jag never had any stopping issues with that one.
Keep in mind the reason U.S. OEMs never used disc residuals is because the master cylinders were all firewall mounted by the time modern (no return spring) discs became available. Production disc/disc proportioning was very common before ABS. Very few vehicles did not have it. Also, most if not all adjustable proportioning valves do not open to full master cylinder pressure. The usual max C/W knob adjustment (knee or crack point) is around 1000-1200 psi., and when exceeded becomes a fixed-percentage of master cylinder, just as lower valve settings. The only way to know for sure you do not need a prop valve is to remove it.
Larry's source car has no factory proportioning valve, for that matter nothing but brake line for the master to the calipers.
No, of course not. But just because the specs aren't identical doesn't mean there's a problem. First it should be noted that the whole purpose of a proportioning valve is to make sure the rear brakes don't lock before the front. Now if you're a race driver, having a driver-accessible proportioning valve can be a good thing, allowing on-the-fly adjustments to account for tire wear and/or track conditions, but for most people it's a set-and-forget deal. Second, I'll point out that Jag used the same braking system (with only minor engineering changes here and there) across their whole product line for years; the '68-88 'series' sedans and '75-88 XJS sports coupe/convertible all used the same brake system. Once the lines left the master cylinder, there was no interconnection; they even lacked the typical pressure-differential brake warning light found on other cars. This was due to a quirk in the master cylinder design, Jag sold the Feds on a 'low brake fluid' light instead. So lets compare specs.... Wheelbase. Jag sold the sedans in both 108" and 113" models, with the XJS coming in at 102". '34 Ford, 112" so that's a wash. I'm going to lump CG and weight together. I'm going to guess that Larry's '34 weight is around 2000 lbs (stock is about 2500). The Jags are all heavy cars, ranging from 3800 to 4100 lbs. Now the easiest path to better performance, be it braking, acceleration, or handling is to reduce weight. I just don't see any problem here either. Tire size. Jag started initially with a 205-15 radial on the six cylinder sedans, the V12 sedans and XJS got a 215-15. They finally bumped them up one size in the '80s. I can't tell what size Larry's tires are, the fronts look to be a bit smaller but at 1/2 the car weight that should be fine, the rears are far larger but again, that will help prevent rear brake lock. No problem I can see. Last, weight bias. The Jags range from 55F/45R (XJS), to 58F/42R (XJ6) to 61F/39R (XJ12). Larry's car will have to be pretty unbalanced to miss one of these... These really are great brakes if properly installed and maintained. In 1977, Bob Tullius in his Group 44 XJS won the drivers cup in TransAm, only missing the manufacturers cup because his was the only Jag racing. Car had the OEM chassis and running gear. In '82, Tom Walkinshaw Racing entered several XJS coupes in the European Touring Car Championship series and was the car to beat through '84.
Why disk brakes release how much system pressure will over ride it? Click on Geo. Alverz continue reading https://www.quora.com/What-causes-a-brake-piston-to-retract
A very grateful and big thank you from all who took time out to reply. A very enormous & special thanks to Crazy Steve just down the road from me for is invaluable reply and PM.
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