1992 XJS BRAKE SYSTEM abS ACCUMULATOR

Thanks. When you are looking at every possible cause, nothing escapes you, and this is a classic case of mis-interpretation. So little time; so much to learn!

The proportioning Valve is used for both the Non-ABS and ABS systems (both Teves III and Teves IV). I do not know if there is a part number difference between them, or why there should be or not.
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Alan, we have covered this many times in the past, here is one example:

In short, big differences. Suffice it to say that one is NLA.

My point was, that on a Teves III ABS car, if one were to retrofit no-ABS brakes, fitting the correct proportioning valve is also in order. Or Teves IV.
I think the pressure in the lines will different and a proper valve needs to be to be safe (you don’t want all the stopping to be only the front brakes, or worse, to lock the rears and spin every time you press the pedal hard). Perhaps your experience is different, I keep contemplating the change to what you offered me and keep changing my mind.

I’m pretty sure my '83 didn’t have a proportioning valve. I never found one, anyway. Anyone know where it would have been?

I stand corrected then:

https://parts.jaguarlandroverclassic.com/parts/index/part/id/C03.900.90058.90058295.900582953762/brand/jaguar/

Will revise my earlier statement.

My point was, that on a Teves III ABS car, if one were to retrofit non-ABS brakes, removing the proportioning valve is also in order. If one were convert to Teves IV, one might consider the correct proportioning valve for that setup

Teves III (item #4)

https://parts.jaguarlandroverclassic.com/parts/index/part/id/C03.910.91058.91058295.910582957898/brand/jaguar/

Teves IV (item #2)

https://parts.jaguarlandroverclassic.com/parts/index/part/id/C03.910.91058.91058295.910582956456/brand/jaguar/

That was the diagram for the front pipes. This is for the rear:

https://parts.jaguarlandroverclassic.com/parts/index/part/id/C03.900.90058.90058295.900582953766/brand/jaguar/

Agree, it does not seem to show a “delay valve”. Seems that idea was introduced with ABS.

:-))

How about front and rear lines (with ABS), for completeness? Notice here the proportioning, aka pressure delay valve, which btw is in the engine bay, is shown as part #20:

My ‘88 does not have one, either.

So, to retrofit the pre-ABS vacuum boosted power brakes, one step is to simply remove the delay valve?

Nope - I’m with Steve on this. Both ABS and Non ABS systems need the proportioning valve of the proper specification for each system. Eliminate the proportioning valve, Step on the brakes hard, and you have a good chance of the back end of the car passing the front end unless you catch it soon enough. Sort of akin to sitting on a wet road at a red light with posi-traction (and no traction control), then stomping on the gas when the light turns green.

There are vehicles without hold off or proportioning valves; MGBs, early Corvettes with 4 wheel disc brakes, and the earlier XJ-S come to mind.
Manufacturers can size the calipers/wheel cylinders to bias the braking to reduce the tendency for the rears to lock, but more modern systems tune this with a proportioning valve, which usually includes a hold off valve to reduce nose dive on light braking. But early XJ-S do not have either.

Number 20, if I am not mistaken, is a residual pressure valve; these are used to maintain a slight pressure on the caliper seals to prevent leakage, keep air from being drawn past the caliper seals, and reducing pad knock back (caused by warped rotors or loose wheel bearings).

Robert,

I had a C3 (1976) factory new OEM Corvette with four wheel disk brakes and it did have a proportioning valve - Likewise my "88 C4 with a proportioning valve. If the XJS’s back in those days didn’t have them as some say, one would have to ask Jaguar why. The '76 Corvette is still in the family, but I upgraded it (2010) with a '96 Corvette LT! / 4L60E, in tank fuel injection pump, Wilwood Aluminum calipers, fiberglass rear transverse spring, Borgeson power steering box and other goodies. Great fun to drive - T tops on (improved modified A/C) or off (natural air).

It would be interesting to know a carmaker’s thinking on proportioning valves. They obviously cost a buck or two, so they wouldn’t put them in the car without a good reason. I hate to suggest this, but it’s entirely possible that somebody sued a car company for not having one, and now all cars have to have them whether they do any good or not.

For you guys who race: Do race cars have proportioning valves? I’m betting they do not; just another something to break. Racers generally carefully set their own F/R brake balance with a balance bar at the master cylinder.

My 1967 Sunbeam Alpine and my 1973 Triumph GT6 – I think it was both, might have been just the Alpine – had no proportioning valves, but they accomplished a similar objective by having a Girling in-line brake booster plumbed into the front brakes only. In theory, supposedly if you pressed gently on the pedal, the rear non-boosted brakes would engage first, preferable if you’re driving on snow or ice. Press harder and the booster kicks in, applying stronger braking action to the front brakes. Both cars had drum brakes at the rear, discs in front.

I don’t believe racers would eschew something, like a proportioning valve, that would have a potential effect on lap times. Efficient braking can be more important than acceleration on some circuits.

On my Alfa, when investigating a brake booster problem, I bypassed it and had unassisted braking. I liked it. When I did a track day for Jag enthussiasts I drove an E type with unassisted brakes. I liked it. In the UK, the MOT test doesn’t allow for changes to the braking system so I’m stuck with what was installed on the Jag. The Alfa is now old enough to skip the test, so I may reinstate the bypass.

I am thinking of a '65-67 that I worked on…it originally had manual 4 wheel disc that had no proportioning valve; I converted it to power assist. And my experience with many MGBs prior to rubber bumpers; they have a pressure differential warning tee that activates a warning light if one system loses pressure, with no holdoff or proportioning valves- 1975 and later MGBs incorporated this into the master cylinder but still had no proportioning valves. The 1988 XJ-S that I currently own definitely does not have one, either.

Braking balance can be achieved by caliper selection; with the XJ-S there was a bulletin that if you switched to metallic pads as used on later models, that all 4 needed to be done to maintain braking balance.

Don’t confuse “trap” or residual pressure valves; they are there no maintain a slight pressure on the seals and to keep the pads/shoes in close proximity to the braking surface. Trap valves are usually in the outlet ports of the master cylinder, but can be in-line- Wilwood offers them in 2 psi (disc) and 10 psi (drum) versions.

Circle track racers have a manually adjustable proportioning linkage. They make adjustments on the fly while racing.