Strange brake pedal characterestic

Ok, here’s an odd connundrum ~20 years in the making.
I’ve got an XK-140 with lots of bits from a '61 E-type, including full brake system (car has IRS), E-type pedals, master cyls, and “bellows” style vacuum booster.

It now has Wilwood front brakes with larger cyls from Dunlop front in rear calipers.

With the booster disengaged the pedal has almost no feel, and only applies mild braking at full travel. With the booster engaged the feel is still pretty mushy, but the braking is fine (not excellent, but not problematic)

It’s strange that I would expect disconnecting the booster to give a firmer feel, not a softer feel.

It was like this when we first bought the car in 2000, before new/rebuilt calipers and master cylinders. It’s just always been like this, and I just assumed “odd but normal for the car”, however reading a bit on others comments here, it sounds like this isn’t “normal” behaviour for E-type brakes, and there’s possibly a problem with the pedal geometry or something to that effect.

I know it’s hard to say since my car isn’t a normal execution, but any ideas where to start?

Hi Brandon, I can’t expound on your brake problem , btu could you provide me with pics of all the enhancements you’ve done to the brakes?

Also I have been looking for a long time for info on a diy for installing a rack into my XK120 FHC. I realize you have a 140 with a rack bu, perhaps, you know of someone who knows how to do tis? Also could you please send me pics of your rack setup as I don’t have anyone local who has a 140 so I can look and take measurements.

My email is: alphajag@att.net

Thanks, in advance, Brian

I’m actually fairly certain I have an E-type steering rack as well… My car was heavily modified long ago (reportedly early 70s) My dad/I have had it since 2000. I will see if I can get some pictures later.

Hi Brando, I’ll be looking forward to those pics.

Thanks a lot.

Brandon, not sure what you are saying. Are you saying the pedal is mushy both with and without the booster connected and you think it should only be mushy with it connected? You do seem to acknowledge that the booster does provide better braking.
If it is mushy both ways, to me the obvious would be there is air in the system. What may not be obvious is that with the changes that you have made, the relationship of cylinders to cylinders to hoses may have just simply introduced some sponginess that is now your normal.
Or am I not understanding your concern?
Tom

Hi Brandon. I written so many articles on modified brakes and given so much advice I’m probably starting to rant. It’s probably the first question I’m asked by E Type owners upon learning I raced an E Type - how do I make my brakes better. My inevitable answer was make sure the stock system is working properly, then add better (meaning bigger and softer rubber) tires. All a brake system can do is to grip the rotor with sufficient force to bring the tire to the point of lockup. Actually exceeding lock up pressure is unnecessary, and potentially dangerous - see ABS braking systems.
Jaguar (carefully I hope) matched piston sizes between the master cylinder and brakes to achieve this goal, and thereby created appropiate pedal movement. By that I mean the pedal would go maybe a quater to a third of the way to the floor maximum to achieve optium stopping. An increase in caliper piston sizes without a proportionally equivilent master cylinder size increase throws this out of whack. It means the master cylinders on you car must move more fluid, leading to a longer pedal, maybe to the floor. This increase in mechanical advantage feels spongy. You may be at the maximum m/c piston movement without actully sufficient pressure in the system to give you near lockup. Wilwood calipers come in various piston sizes - do you know what they are and their relation to the original size of the E Type front pistons? Now with bigger rear pistons the problem is compounded. It’s simple to do calculations and see how far the m/c pistons are moving, if you know the caliper piston size.

Yes, I absolutely understand that portion of it. The Wilwoods I have are the 4 piston set sold by ClassicJaguar circa 2003, “billet dynalites” I think.
However I had noticed this before we changed calipers, and it was neither better, nor worse with the caliper change. Air in the system seems most likely, but it’s all been bled a few times. Need to try again.

What baffles me is that with the booster hooked up I get much better braking for the same pedal travel. I would think that brake system pressure would be a function of pedal travel regardless of booster being hooked up? I.e. with the booster hooked up and working correctly a slight brush of the pedal will apply mild braking, but without the booster I’m about 90% pedal travel to get the same amount of braking. If the main problem was just air in the system, it seems like braking would be poor with or without the booster?

With or without the booster, there’s very little “feel” to the pedal. It’s as though the booster primarily amplifies the travel rather than assists with additional force…

I may have just uncovered the issue, in quickly looking at the master cylinders with this in mind, though I’m not intimately familiar with master cyl innards.

The exit from master cylinder to the brakes is on the bottom of the cyl. As air usually rises, it seems to me like it would be very difficult, perhaps impossible to get air out of the master cylinder. From the diagram on SNG Barratt site, it looks like the normal fitment has the output going more or less straight up. I am deducing correctly here, or would the piston travel be sufficient to push any air out regardless?

Hi Brandon The diagrams I have of the bellows type braking system show that your m/c’s are upside down so you certainly have a point about bleeding.

Brandon, if the cylinders are inverted as Terry says, and the outlet should be on the top, it may help to bleed from the bottom up to push the air out, although that still might not get it all out.
Tom

I’m just thinking since air usually rises to the top, the normal method (output at top) would tend to push air out through the main brake lines, while this would leave some amount of air permanently, unless the piston can travel sufficiently to drive it all out. I think it could be close if I bled them off the car, but probably not perfect. May be better off to rotate them, though I’ll need to extend the hard-line for that.

Oddly, I’m 90% sure this was all replaced ~2004-2005 by a reputable Jag specialist so it seems strange it would be done incorrectly.

Nothing is strange in this world, many specialist can get things wrong, be very wary if you use an expert (=x an unknown quantity, (s)purt a drip under pressure) said with the utmost respect to those specialists who actually know what they are doing and talking about.

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Dynalites come with three different piston sizes - the total square inches of each caliper being 6, 8.24, and 9.60 representing piston sizes of 1.38", 1.625" and 1.750". The total area on stock 2 1/8" Jaguar pistons is 7.1 square inches per caliper. If you have 1.750 pistons in the Wilwoods you are pushing a lot more fluid 35%, if 1.625 pistons (most likley size) you are pushing 18% more. Going from 1.750 rear pistons to 2 1/8 means a 47% jump in total fluid flow to the rears.
Of more concern is that assuming your front pistons are 1.625 going to the 2 1/8 rear pistons changes the front/rear braking ratio to 53/46%, which is getting concerning. You will need to test the car ona safe surface to ensure the the rears don’t lock first - an extremly dangerous situation.

That sounds like a pain, which is why I suggested you may want to simply bleed from the bottom up first to see if that would help by getting a larger percentage of the air out.
Tom

At the time we did it (2002-3) the 2-1/8 to rear, with Wilwood fronts was the standard/recommended setup. Could probably track down what Wilwood was used but anyway not the immediate problem.

Incidentally, I did lock up the brakes on my last outing and had no issues maintaining control.

If you’re interested in some of the previous discussion: