Teves III convert back to vacuum booster - questions

As part of my ongoing 5-speed conversion, I am thinking I want to toss the Teves III over the hedge and go back to the 88-earlier vacuum booster / master cylinder set up. The III is not really rebuildable, probably dangerous due to age or design, and I need an accumulator ball which is $300. And I don’t really care about anti-lock, so I am not interested in going to the cost/hassle of converting to Teves IV.

I have a Teves III pedal box with a clutch pedal, so I’ll fab an adapter plate that will allow me to bolt the vac booster to the front of that pedal box. Questions for the group:

  1. Does anyone have a template or measurements for the base of the vacuum booster, i.e. location of studs and center hole? There is a gasket that goes in there, “Seal - Base C39597” that would be perfect, but they seem to be unobtanium despite what the internet says. I’d like to mock up a bit before committing to purchasing the actual vac booster.

  2. Do I really need the vac reservoir tank? Most other cars don’t have them, and it seems like the v12 should pull plenty of vacuum with the butterflies closed… but Jag put the reservoir in for a reason. Does anyone have any real-world experience with removing the reservoir? I guess I could build without and then add later if I am not happy.

  3. Is there a proportioning or front/rear bias valve? Most cars have them, my parts manual does not show one.

  4. What else am I not thinking about?


Good luck! I love the simplicity of my 1988 Brakes with just a booster and master cylinder.

BUT, I did have one incident a few weeks ago where somebody cut me off and I had to slam on the brakes. Boy, I forgot how much tire smoke can be generated when you’re sliding to a stop!


Hi Bob,
Regarding #3- an old paper catalog I have shows a bias valve - P/no C-41668. Good luck finding one, perhaps you could go generic. FWIW, my car (88) does not seem to have one, unless it is buried somewhere out of sight.

1 Like

If in Jaguar language this is “pressure delay valve”, your car should have one (part no.4).

You are not planning on changing every hydraulic line, correct – just ditching the pump/accumulator and swapping the ABS block with a vacuum booster, are you?


PS I am having problems with the image, this is the URL:


Thanks Steve-
My car probably has one of these for the Teves III system, but I am wondering more about the pre-88 system. I shows nothing in my book for pre-88. I may put in a Wilwood adjustable valve for grins. Nice to have the option and won’t hurt anything.
I am not changing hard lines unless I run into weird threads or need to bend up some new ones to align with the master cylinder.

Interesting. Moss Motors shows this PN as XJS proportioning valve 74-87, NLA.

My parts catalog is labeled “JAN 87 ON” at the top of each “fiche”… maybe that’s why I don’t see it.

The vacuum booster is a standard GM part. Hence, the gasket probably is too – which is why nobody bothers to stock it under the Jaguar part number.

Good question. I suspect the answer is no, if you’re strong enough to stop the car with the brakes unboosted. There is a tiny chance that not having the reservoir will affect how the engine runs, like perhaps a stumble or something when you hit the brakes, but I kinda doubt it.

Y’know, you don’t need the Jaguar vacuum reservoir. Any airtight tank will do.

My '83 did not have one, despite one being mentioned in the manuals. The only thing my car had was that block that registered a system failure and lit a warning light on the dash.

As always, I recommend a Mitsubishi brake fluid reservoir rather than the Jaguar POS.

Recently, Norman Lutz has explained why he believes the brake balance is poor on the XJ-S, the rears are too weak. If you accept his argument, the car will not stop as well without ABS as it would with, as ABS modulation would permit full application of all the brakes on the car. His idea for correcting the balance is interesting, just bore out the rear calipers to use the pistons and seals from the front calipers. You might consider doing that – but of course it could be a separate project later on.

I have a couple accumulator balls if you need one.

I’m betting the vac reservoir is only necessary for braking during an “engine off” situation. The brakes still work fine, just very heavy pedal. Been there done that not worried. I am going to go without for starters. Easy add later and many aftermarket options.

I will add an adjustable proportioning valve. The only downside is that it adds a couple of additional connections.

I assume there is zero clearance for an integral master cylinder reservoir. I will get the pedal box and booster in place and see, would love to be wrong about that.

The big unknown at the moment is whether there is clearance for the vac booster once it’s located on the teves pedal box. These are two parts that were not meant to go together. Wondering if the “centerline” of the teves pedal box is lower than non abs.


If you have outboard rear brakes then you have more options available off the shelf from XK8, XKR or Aston Martin parts. I have XKR calipers and vented disks on the rear of my XJR-S.

Why? They typically weaken the rear brakes, which Norm claims are too weak to begin with. Seems just the addition of another possible failure point to me.

So, you’re planning to use a non-Jaguar master cylinder?

I dunno if there’s enough space. Some of those cast iron American master cylinders with integral reservoir look pretty low, I’ve wondered myself if they would work. They’re dreadfully heavy, of course, but IIRC they do have a properly designed diaphragm to keep air away from the fluid.

You probably want to pay attention to the bore diameters. Smaller bores in the master cylinder make for less force needed on the brake pedal but more travel.

All good questions, especially as your Teves pedal box includes a clutch pedal. If all else fails, you could seek out a Teves IV pedal box, as those presumably were available with clutch pedal and worked with the standard vacuum booster. Even if you can’t buy one, just getting a look at one might be helpful. Note: They are reportedly made of plastic.

I’d prefer to use aftermarket with integral reservoir. It’s brand new, actually cheaper than the Jag unit (Jag unit WITH the silly little plastic elbows, which I need, are close to $300) and much simpler. There is currently plenty of room for my Teves reservoir right on top of the MC, but of course the booster pushes everything forward where it gets tighter. This particular unit is 15/16 bore, same as Jag AFAIK.
Worst-case I will go with stock or stock-like setup with remote reservoir.

If I have to get another pedal box I would likely go backwards and find a pre-abs unit. Pretty rare and pricey. I am pretty confident that I’ll be able to adapt my current setup. The centerline issue aside, the next concern is that the vac reservoir has studs on it, which are meant to go thru “ears” in the pedal box and have nuts put on them. The Teves III pedal box has studs of its own. They are threaded in.
The working theory is that I will be able to build an adapter plate out of say 1/4" steel, drill the pattern for the the vac reservoir, and drill a second countersunk pattern for the existing pedal box. Bolt plate to pedal box, bolt vac reservoir to plate. Similar to a transmission adapter plate. But I think there will be interference with at least one vac reservoir stud. Worst case, I am fortunate to have good access to machine shop and expert TIG welding. This can’t be that complicated :rofl: … famous last words.

I ordered a vac booster, will know a lot more when it arrives in a few days.


I’m really looking forward to seeing how it goes for you. If it works out for you, I’m sure quite a few people would be looking at the mod. I’m not a big fan of the no brake situation with the teves3, been there, didn’t like it. However, I must admit that I am also not a fan of locking them up in an emergency situation and sliding into stuff. Lol :grin: been there done that too :grin::pensive:

One of those parameters can be fixed by proper training, the other, not so much!


I got a quality rebuilt MC for $150. And if you mean the silly elbows between mc and reservoir, I ditched them and used that special see thru hose. Reservoir, you could get mitsubishi as stated in the book.

Greg, post a pic, please. I see no-elbow MCs for $150 or less, but I thought the elbows fit into some sort of special hole and were thus required. Are you saying there is just a hose nipple under them, or what?

you mean those plastic elbows that fit in the top of the MC? I actually ordered new elbows, I’ll check my records tomorrow and post back. They were only $10-20 if I remember.

1 Like

Greg can you please post the same pic, except to move the camera back towards the firewall so I can see how the booster mates to the pedal box exactly? Thanks!

Here’s what we know from Greg’s picture: If the Wilwood master cylinder with integral reservoir won’t fit, it won’t be because it hits the hood. It’ll be because it hits that diagonal strut. So, anybody got any ideas? Perhaps replace the diagonal strut with one that’s flatter, allowing more space?

Or, considering he’s removing a Teves III, perhaps there’s a way to space the master cylinder forward to get it out from underneath the strut?

Greg, that one reservoir hose looks tight! Why didn’t you cut it a bit longer?