In reply to a message from Grooveman sent Tue 20 May 2014:
I did this conversion a while back at age 73. No problem
provided I planned well and was careful. One of the best
things I ever did with the car. One hitch I had and that
was that the booster (ex Series III) would not quite fit
against the engine bay wall. I had to hammer the wall in a
bit with a hide hammer. I used the kit from the US. A high
price but good stuff. The booster I had had a non-return
valve in it hence not required to site one in the hose.
Essential to have one somewhere else no power brakes after
engine stall! Dangerous!ā
Alan (XJ40 3.6L 1987/8)
Salamander Bay, nr Newcastle, Australia
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Thanks for the tip Steven.
I have checked that write-up from the XJ40 Ebook on the Jag-lovers site. However, I still wonder what other brake boosters could be used besides the XJ6 Series III booster.
I wondered the same thing so I decided to experiment with a different booster when I did this conversion year ago ā¦ sadly !
I purchased a dual diaphragm 7" booster from Summit Racing, which was a little smaller diameter (making the fit easier) and of course with the dual diaphragm it would give me greater stopping power, right ā¦ WRONG ! No matter how hard I pushed on the brake pedal I couldnāt lock up the brakes.
So I learned my lesson and purchased a Jaguar Series III booster, fabricated another new adapter plate and have been pleased with the results. Not to say there isnāt a better booster for this job but with literally hundreds of these conversions done over the years Iāve never heard of anyone using anything other than the Series III booster, so that tells me something.
It is also interesting that at least some of the Series II (probably Series III, too) boosters include the check valve (RTC1128). It says in the Conversion kit instructions that you need to acquire āvacuum inline check valveā. I think that this is not necessary if the booster already has one.
Iām not sure if there is a difference between the series II and III vacuum booster. I couldnāt find any info on that. Why do you want to experiment and use a different booster, the series III one is a proven commodity.
As far as the vacuum check valve. The braking system will work just fine without one ā¦ as long as the engine is running. What the check valve does is give you a few assisted pumps if the vacuum source (engine) shuts down. Donāt confuse the elbow out of the booster as a check valve, some are, some arenāt. In my case below the red fitting attached to the booster is simply an elbow and I had to add the light grey in line check valve.
Thanks for the clarification, Grooveman.
How can you tell whether that red elbow is a valve or simply an elbow for hose attachment? Probably one could just put another valve between the hoses to be sure. Maybe the system would work with two valves.
Just pop out the elbow (itās just held in by friction with the rubber grommet) and blow thru one end and then the other. If air moves freely thru both ends itās just an elbow. A check valve will restrict the flow of air going thru one end.
I donāt have the new booster at hand yet. However, I think the diameter of the elbow/valve is 13 mm. Anyway, the diameter of a separate check valve is 13 mm. So, maybe it is good to use a hose with 13 mm inner diameter and a suitable hose fitting which will be screwed into manifold. I am just a bit confused since some people have apparently used 1/4" and 11/32" hoses.
I have now removed the hydraulic equipment and prepared the pedalbox/booster assembly and the HSMO pump blanking plate for the installation. I used 6 mm thick aluminium for the booster mounting and pump blanking plates. Instead of using the original clevis pin and clip, I used a 8 mm bolt and a nylock nut to attach the booster clevis and brake pedal. Next steps are drilling and tapping the manifold for the vacuum hose and then installing the pedalbox/booster assembly back to the car. It seems to be a very tight fit, so most likely it is necessary to tie the master cylinder in order to keep it enough away from the booster during the installation.
The booster I am using is from a XJ6 Series I car. The colour of it is grey. I donāt see why it would not work even though Series III boosters are used usually. Someone used here a similar grey booster even if it is stated in the text that a Series III booster was used. Probably some cars of the Series III had a grey booster. https://www.jaguarforums.com/forum/xj40-xj81-25/xj40-brake-booster-85830/
It seems that probably the only difference between the XJ6 S1 and S2/3 boosters is the four rear mounting studs. The studs of the S1 booster are imperial size (inch/UNC or similar) whereas the studs of the S2/3 booster are metric (M8). Can someone who has a S2/3 booster confirm that the rear studs are metric (M8)?
I bought slim M8 nuts for the S1 booster but found out that I had to use the original imperial size nuts which came with the booster. I made one of the nuts more slim in order to attach it to the 1 oāclock stud of the booster. I did not attach the lowest 7 oāclock stud to the mounting plate with a nut because it is too tight down there.
I finalised the conversion. After the first start-up the brake pedal was very stiff. I wondered what could be the reason and concluded that probably the bolt and nylock nut I used to connect the booster to the pedal was too tight. I put the original clevis pin and clip and the brake pedal felt very good. However, I noticed that the brakes drag. Maybe the brake booster push rod length is incorrect and/or the master cylinder is overfull of brake fluid. I have not added brake fluid after the conversion so maybe the latter would be a strange cause for the brake lag.
I used the same idea as you did ā¦ a bolt and a nylon lock nut rather than the original clevis pin, much easier to install. I doubt that was your problem unless you REALLY tightened it down. The long brake pedal gives a tremendous amount of mechanical advantage. But since you put the clevis pin back in and it now feels very good who am I to argue with success.
Jack up you front wheels and adjust the rod going into the brake booster between the master brake cylinder. You can actually make a cardboard gauge to do this with. In the end you should have a normal pedal throw and be able to spin your front wheels and not feel any drag.
Even if the master brake cylinder is overfilled it wouldnāt cause your brakes to drag.
Now the problem is that the brake booster actuator rod is jammed into the master cylinder. I try to make a half-cut pipe which would hold the master cylinder piston in place so that I could move the rod towards the brake booster using pliers.
Do you mean you canāt physically separate the booster and the master brake cylinder? I donāt remember the actuator rod going far enough inside the end of the MBC to get stuck.
Yes, the rod was stuck in the master cylinder. I had to work to get it out. Now the challenge is to adjust the nut. I sprayed some WD-40 and hope that I can turn the nut later.
Is the rod rusted on? WD-40 wonāt help very much with that. Better to use penetrating fluid.
If I remember correctly the small rod just screws in and out with finger pressure for adjustment. Iiās such a short length I canāt understand how it could get stuck in the master cylinder?
Yeah, it is strange that the rod was heavily stuck in the master cylinder.
I removed the pedal box/booster from the car, pulled the rod from the booster and screwed it very tightly to the bench. I could open the adjustment nut with pliers. The thread was luckily in a good condition. I re-greased the rod and the nut and put the rod back to the booster. The next step is to install the pedal box back to the car and adjust the nut so that the brakes wonāt drag. Luckily, I can move the nut by hands now!
