Hi all,

Having fitted 420 front callipers to my Mk 2, I assumed the rears would need to be similarly upgraded for reasons of brake balance. I also saw a similar comment to this effect on the Jaguar World forum.

Interestingly, the original 2 & 1/8" cylinders + pistons from the front bolt straight onto the rear callipers & the pads are also the same size.

What a breeze, I thought. After getting the missus to pump the pedal & help with bleeding, I ended up with a soft pedal & excess travel.

I can therefore only assume this conversion DOES NOT WORK!

My theory is the master cylinder/piston size does not have enough capacity to service the larger calliper pistons at the rear, even though a 420/E-type MC is fitted. (Note to self - should have thought of that.)

Back to fitting the original pistons it seems, with a lesson learnt - let sleeping dogs lie!

 

Have you tried leaving the original rear 1.5 inch calipers in place ?

 

I don't think your prognosis is correct, after all the volume of fluid movement is minimal, and should be well within the capabilities of a 420 master cylinder. First off, new pads need to bed in, and pedal can be spongy for a little time after fitment. Second, if there is disc run-out, the pads will be backed off too much and hence more pedal movement will be necessary to get the pads onto the discs.


FWIW these early piston/cylinder designs have a rather complex pad withdrawal mechanism. Remember they only came into production very shortly after discs has been developed by Dunlop. Much effort went into ensuring the pads were positively withdrawn from the disc but not too much to avoid excessive pedal movement.
Modern pistons rely on the piston seal flexing to provide pad clearance when brake pressure is removed, but these early pistons have a rod secured at the back of the cylinder that engages in a "clutch" in the piston. The rod is fixed to the cylinder using a washer that flexes under brake pressure, and then on removal of pressure, draws the piston back off the disc. The clutch has a much tighter grip, but as the pad wears takes up any slack in the movement.


If it were me, I'd check the discs for run-out first before reverting to the old size pistons, and make sure disc surfaces are flat and not scored.

 

Thanks Jeff & Fraser for the replies.

I think once again I have jumped the gun & selected gear with brain in neutral.

I re-bled the brakes & got a few more bubbles out and the pedal now feels a lot better.

Pedal travel is a bit more than I would like, but I can reduce this now that I have fitted the 420 model's pedal stop adjuster.

Jeff, will road test the car & consider re-fitting the 1.5" pistons should the braking proves to be no better. I found out these cylinders were reconditioned with s/steel sleeves by the previous owner, so I have had a win there, at least.

 

Pete, I fitted the 420 front sub frame with the varimatic steering and upgraded to xj6 calipers. I was told that I should leave the old rear cylinder as after seeking advise from a JEC person,as I was thinking of fitting xj6 rear calipers, he recons that the rears might not balance otherwise!!!! . I have not done a lot of miles but braking is a lot better than I thought it would be. I drive a volvo daily, so you compare with that, and although you need alot more pedal push I thought them unexpectedly good. the brake assist does a lot to, brings the affective pedal travel to the top, really firm, with little travel
By the way whats the 420 pedal stop adjuster?
BAZ
PS you can get 3 sizes of bore in similar pattern master cylinder.

 

Hints about bleeding (just some theories from my own experience).
Bleeding the brakes by pumping the pedal can aerate the brake fluid by breaking the larger air bubbles into smaller and smaller ones always leaving a spongy pedal.
This is also a nuisance because it requires two people and I find that I'm never really quite sure if all the air is out.

Solution one is to get an inexpensive vacuum pump to easily suck the fluid through the systems the bleed nipples.
One annoyance with this is that the vacuum pump also sucks air past the some-what sloppy threads of the bleed nipples making it appear that there is still air in the system.
To solve this I wrap the nipples with Teflon tape to seal the threads off. This also has the added benefit of keeping water and dirt out of the threads so the bleed nipples don't become welded in there for future brake maintenance. I've done this method on my little Mazda B2200 when I replaced the master cylinder _ works really well...

Solution two is probably the best one and the one I'm going to use.
Rig up my air compressor to the plastic brake fluid reservoir and apply 1 or 2 pounds pressure (shouldn't take much) to the system.
This should gently force the fluid into the system and push the air out through the bleed nipples.
I shouldn't even need to bench bleed the master cylinder with this method.
I plan to start with the front calipers first until the master cylinder and booster are full of fluid and then move to the back.
All I have to do is make sure and keep the reservoir topped up.
I'm looking forward to trying this.

 

Jeff, your choice, but you usually start bleeding the furtherst away from the M/cyl first
There is a system , inexpensive, called Eastbleed (in UK)that uses pressure, runs off the spare wheel, same really, effective, accept for sloppy threads
Barry

 

That I didn't know about starting at the nipple furthest away from the MC _ thanks.
I've never tried this before and when I replaced the MC on my truck I can't remember where I started...


Here's the rig here _ pretty cool _ might just go with this because it looks like it has a lid that fits the reservoir.