Thanks. I was over there today on the accumulators and pumped and pumped, and it takes some effort, but it still goes to the floor, and I am thinking master cylinder failure. I never liked this system, and got all kinds of mixed messages on the brakes. I tried the pedal with the car running and car off, and no difference. When running and pumping the brakes, there is a light on the dash that looks just like the accumulator image with a cross across it, as if to tell you that is the problem. What next?

As to retro-fitting the vacuum booster and ordinary twin master, I am wondering if there is enough room for the booster setup and saw one conversion on a RHD car, and it fits, but mine is a LHD model. The booster itself is reasonable in price and most any master would work, providing the mounting holes line up with those on the booster. I know the front brakes have separate lines, but a ‘t’ fitting would take care of that. Let me know your thoughts on this, and thanks

This almost sounds like you are describing the “Bulb Failure” indicator light - could possibly be indicating that you have a failed bulb in the brake light section - Tex Terry, II - 1991 XJS V12 Classic Coupe, 1986 XJS V12 Coupe - sent 7/30/2020 0005hrs. EDT USA,

Probably not. With the vacuum booster and the master cylinder in place, the forward end of the master cylinder sits just below the diagonal brace. I dare say most generic master cylinders – which typically have a built-in reservoir – will interfere with the diagonal brace. That’s probably why Jaguar went with a remote reservoir.

Retrofitting the earlier non-ABS vacuum boosted power brakes in place of the Teves III should be fairly straightforward utilizing the Jaguar master cylinder, a Mitsubishi reservoir, and a GM vacuum booster. The hard part is that the job absolutely requires the pedal box from a pre-ABS XJ-S; the pedal box from an XJ saloon won’t do.

As I responded to your PM - you need to make sure the supply line is belt properly and the pump is running.
The Teves III ABS system is one of the most frequently discussed topics, as the understanding of what it does and how it is operating is not good. Here is some relevant information:

In short, if you CONFIRM that the front brakes are free of air and there no leaks - engine and ignition off - brakes should operate as conventional brakes and can be bled as such - then the problem is in the solenoid block/master cylinder.

In theory, conversion to an old-school vacuum boost should be fairly straightforward, but there are many details that need to be worked out and some of the brake lines need to be reconfigured.

I don’t recall what the situation with the proportioning valve for the rear brakes was (it is also frequently discussed here).

Paging @BobPhx
He did the conversion (couple to a modification from auto to 5-speed manual) and documented the experience here:


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.

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:


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)


Teves IV (item #2)


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


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).


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.