1989 XJ40 vacuum assisted brakes

(Grooveman) #1

Season greetings fellow Jaguarist ,

As the weather grows frosty and the holiday season is in the air our thoughts naturally turn to braking … what … BRAKING ?

I thought I’d throw something out and afford myself of some of your high powered brainstorming.

Years ago I modified my braking system from the factory Hydro braking system (that was ALWAYS leaking) and gave me decent braking … to … a vacuum conversion that was professionally done (by me) and is simplicity in itself and never leaks but still only affords me decent braking. Actually the braking will put you through the windshield, it’s the brake pedal pressure required to do that that I’m not particularly happy with. If I could get 20% more actual braking per pedal pressure than I do now it would be perfect.

I’m wondering if there isn’t a more robust front brake caliper available that could replace the single piston one that’s installed. Are the XJ-S calipers any beefier? would they fit?

Would a different/modified master cylinder produce more PSI at the calipers with the same pedal pressure?

Just some thoughts I’ve been pondering. I hate something being “almost” there.

Oh yes … HO HO HO

( Larry ) #2

Well mine (thoughts that is) actually turn to batteries rather than brakes, especially as the old girl refused to fire up this morning :weary: …got a jump and took her over to the battery shop to get the battery tested …verdict? 3 yr old battery worn out.
$180 later, I have the battery of batteries, managed to squeeze a usually-fitted-to-diesel-trucks 850CCA sucker in there so I should be good in the starting dept. for some time to come, methinks.

Hate to be stuck out at the lake with a pile of trout and no way to get home, so don’t mind coughing up the $$ for peace of mind.

Anyway, I could probably do with new brake sphere but I can put up with the odd blink from the abs light till next spring …brrrrr! :snowflake::snowflake:

On the 94 with the Teves system, you don’t really have to stand on the brakes, she seems to stop pretty well with normal pressure - sorry that’s about as much as I can say brakewise …so seasons greetings an’ all to you too and good luck with the brake stuff! :christmas_tree::christmas_tree::snowman:

(John Quilter) #3

Is the anti lock feature still retained when you convert to vacuum assisted brakes?

(Grooveman) #4

John …

Yes the anti-lock system is still functional after the change.

The conversion is very slick, and works like a charm! Actually the required pedal pressure is almost the same as it was with the hydro brake system, really not much different. I’ve always liked a bit more sensitive pedal though.

I’m seriously thinking about getting a used brake pedal and reducing the distance of “Y” (where the output rod connects and the pivot point). That should give me more output force with the same pedal pressure. There are all kinds of calculation charts on doing this available on line. Sounds easy but I’m sure there are problems to overcome.


(Pete55Tbird) #5

My very imperfect understanding of brake systems tells me the size ( piston diameter ) of the master cylinder is
the important part of OMPHH to apply your brakes, Check it out. Pete


(Grooveman) #6

Pete …

You’re absolutely right the size of the brake caliper piston is one of the key factors along with the size of the master brake cylinder bore and the brake pedal ratio.

I decided to educate myself on this subject (one of the many benefits of being retired) and was shocked at the amount of information and research that’s available online.

I just purchased a used brake pedal assembly on EBay for some experimenting. So I think I’ll work on the brake pedal ratio first. I mean HEY … what could possibly go wrong.


(Andrew Waugh) #7

Worst idea I ever heard… when do we start?

The problem with changing the pedal arrangement is twofold:

  1. What you gain in force, you pay for in travel.
  2. The damned thing is mounted in the footwell, so you end up laying down in the footwell.

I’d be more inclined to look at the hydraulics side. One option would be to change the diameter of the M/C, the other would be to fit a booster with a higher ratio.

I’d be inclined to start by fitting a booster with a high ratio, but add a regulator on the vacuum side to give some adjustability.

Also… what pads are you running at the moment? Pad composition and age make a big difference to braking.

(Grooveman) #8

Andrew … All very good points my friend.

I’ve found that dealing with this subject is literally like juggling Jell-O. I mean there are so MANY variables.

Booster …
I tried 2 different vacuum boosters. The first was a very neat looking dual diaphram 7" booster from Summit Racing. It fit perfectly but just didn’t provide enough braking power, no matter how hard I pushed the pedal.

The second booster that I am now using is from a Series III XJ6. Apparently this is the booster commonly used in the XJ40 vacuum brake conversion. The braking power is fine, you can lock the wheels if necessary. The pedal pressure however is just a bit higher than I would like.

Engine Vacuum …
It seems that 18 PSI of engine vacuum is generally what’s called for to operate brake boosters. I’m only reading around 15 PSI. Why, I don’t know. My cylinder compression readings are very good ! I wonder how significant 2 or 3 PSI is on the booster performance?

Brake pads …
Years ago I switched to EBS ceramic “Red Stuff” pads on the front wheels (not available for the rear) and although they are a tad pricey they have excellent reviews and I’ve bee very happy with them.

Master brake cylinder … My research shows that the smaller the MBS bore the greater pressure it provides (but also the less fluid it moves). But where in the world could I find a smaller bored MBS that would fit ?

Brake Pedal Ratio … as discussed before this mechanical change would reduce required pedal pressure but as you pointed out also reduce available pedal travel.

As soon as my used pedal assembly arrives I’m going to start there and experiment. It looks like I’m going to have to physics the hell out of it to figure out a new pedal ratio. As you so insightly stated… “Worst idea I ever heard… when do we start?”

(Andrew Waugh) #9

15/18 is a 17% difference, but I’m not sure how much difference that would make to the servo. You could swap the engine with a known good one then we’d have some empirical data.

I’m not sure if just reducing the diameter of the M/C would do the job. A voice in the back of my head says you might need to also reduce the diameter in the servo cylinder as well.

If all you want to do is change the pedal effort, then see if you can arrange two turnbuckles on the “new” pedalbox, top ends on a longer pivot pin at the top, and the bottom ends joined with a trunnion linked to the clevis to the M/C and resting on the back of the pedal drop arm. This would let you try moving the clevis point around until you’re happy with the effort, then you can drill the drop arm. The pedal will be a bit higher up while you’ve got the turnbuckles in but it would be worth it to testbed the idea.

(Grooveman) #10

Thanks again for your input, very appreciated.

I think your advice on the brake pedal is the direction i’ll be going. With the spare pedal i’ll be able to experiment and if worst comes to worst (:astonished:) I can always put everything back the way it was !

Swap the engine !! … begone Satan.

(Grooveman) #11

A bit of a Holiday update on my brake pedal project …

Well my EBay brake pedal assembly finally came in …

Very interesting ! As you can see from the picture the brake pedal ratio is only 3.58. Why is this interesting, well the normal pedal ratio for a power assisted system is between 4 and 5.1, (and for an unassisted, manual brakes, system between 6-7.1). Obviously the original Hydro-assisted system produced more force per pedal pressure than the vacuum assisted conversion does.

Just doing some simple math tells me that if I can get the brake pedal ratio within the recommended ranges
I believe that will reduce my required pedal pressure by around the 20% that I’m shooting for.

Of course the simple solution would just be to drill a new hole in the pedal 1" closer to the top pivot point which would reduce the distance between the top pivot point and the brake booster rod attachment from 4.75" to 3.75". This would result in a 4.5 brake pedal ratio, right in the middle of the recommended range… but … that would also drastically increase the angle the pushrod enters the brake booster. CAN NOTHING BE SIMPLE !!

(Mike Stone) #12

Grooveman, you are way in over my head here! I hope Santa brings you the solution you are seeking. Merry Christmas to one and all!

(Grooveman) #13

Mike …

You’re absolutely correct, to the average forum dweller just getting their car to start in the morning and keep running is a lot more relevant than my brake pedal ratio goobledgook!!

…But… I just can’t help trying to make things more perfect-er :grinning:

(Mike Stone) #14

I think it’s great that you are exploring the edges of the envelope, so to speak. The world needs more forward thinkers like you. :sunglasses: