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Dear all,
I am restoring my 1966 S type and am currently rebuilding the Front subrframe.
I bought a used subframe including brakes and power steering from a 420 Sovereign and so have the three piston calipers;
I've never rebuilt calipers before so am looking for any advice people can give me.
The pistons are seized.
I see that pistons and a seal kit are available from SNG.
How do I go about this? Soak in a bucket of diesel or acetone to loosen things up then use compressed air to drive the pistons out?
There's no cross over pipe so is splitting them a No No?
Any comments gratefully received.
Quite often trying to push the pistons out with compressed air does not work so I set up a jury rig which was an old brake pedal and master cylinder which I am able to connect up to the calipers. Once the system is bled and filled with brake fluid the pistons pop out fine. One of the things you have to do though is use some form of clamp to hold the pistons just on the edge of popping out. If one pops out before the others start moving you will have to reinsert the popped out one and start again. The cross pipe would not need to be fitted as long as it is removed and the jury pipe can be screwed into each side of the caliper but on the double piston caliper you again have to clamp the first piston to move just short of popping out to get the second one to move.
On another Jaguar forum one of the members stated that he used a grease gun to force the pistons out. Not sure how he connected the grease gun to the caliper but one could easily work something out.
The repair kit consists of new rubber seals and they are easily replaced once the pistons are out. Splitting the caliper is the easiest way of cleaning them up and getting the pistons back in. You can buy a small honing device which fits in a drill to clean the bores out. https://www.ebay.co.uk/itm/262571766...3ABFBMgLiUvJZf
I have not taken these particular calipers apart but if they are built like the Ford Capri calipers where they have an internal oil way between the two halves there is normally a small round rubber seal that sits over the oil way. If there is just the one entry for the hydraulic line on one side of the caliper I would not split them until you have used the hydraulics to push all three of the pistons as far as you can out of the calipers. Then when you split them you should be able to use compressed air to remove them fully.
I suggest that you have them stainless steel lined. I had all master cylinders, slave cylinders & calipers stainless lined. Then you have no worries for the future. Brakes will last forever in a relatively low use car other than maybe a seal failure. All is done to standard spec so that original dimension rubbers etc. can be used.
Some use brass which is also OK just generally less durable to mechanical wear but will make little difference in this application. A 420 front end is a good move.
Just looking at parts;
From Martyn Robey in the UK new pistons seals and bleed screw are 30 sterling per caliper and a fully refurbished caliper with a core return is 90 sterling;
Just looking at parts;
From Martyn Robey in the UK new pistons seals and bleed screw are 30 sterling per caliper and a fully refurbished caliper with a core return is 90 sterling;
So an extra 60 sterling each for a professionally rebuilt unit. I might just go for this option I think
In view of the safety issue, I think this is the best option, IMHO. I think one can also buy modern, (new) calipers that just bolt on, but more money, obviously. When I did the brakes on an XJ6 in the mid-90s, I replaced all of the calipers with exchange, rebuilt items. I also replaced the master cylinder with a new one too, but you could still buy these easily then.
This type of Girling unit does not need to be relined. What may well be needed is new pistons as the piston surface provides the sealing. As said there is a special seal between the halves. As far as pumping out the pistons this has little success as the least resistant piston will release and at that point no pressure may be built. Success usually involves soaking and some spinning of the pistons while pulling up on the piston lip. It is easy to damage the piston but as said earlier they most likely need replacing.
Please explain. While pistons are shallow they still fit in a bore. The manual still suggests that any scuffing, abrasion or corrosion of the bore is undesirable. Obviously very light damage can be cleaned up. The piston still has to be sealed to the bore. If there is a deep rust pit in the bore the unit will still leak.
Last edited by Glyn M Ruck; Oct 25, 2021 at 10:18 AM.
In my experience compressed air to remove stuck pistons can be somewhat violent. A safer method is to use a grease gun to apply pressure. somewhat smoother and safer IMHO
The seal fits in a groove inside the bore. It is the piston finish that is important. The pistons are often rusty (chrome finish has been destroyed). Clean the bore---yes. Sleeve it--no reason to.
The groove on the pistons is where the external dust seals fit, it is not for the hydraulic seal which is fitted into a groove machined in the body of the caliper. Essentially, it is totally the other way around to the previous, (original) caliper design where the hydraulic seal was on the piston. The later design is absolutely miles better.
The groove on the pistons is where the external dust seals fit, it is not for the hydraulic seal which is fitted into a groove machined in the body of the caliper. Essentially, it is totally the other way around to the previous, (original) caliper design where the hydraulic seal was on the piston. The later design is absolutely miles better.
Thanks Fraser. Much better. Fully understood. Advanced for the time. Old hat today.
Last edited by Glyn M Ruck; Oct 25, 2021 at 07:10 PM.
I just picked up a rebuild kit for my 420 three pot callipers that i'm fitting to my MK2, these include new bolts, stainless pistons and the seals between the two halfs and were only £60 delivered.
I decided to refurbish and have ordered the rebuild kit detailed by Homersimpson which includes the internal crossover seals and new bolts.
I used a grease gun to drive out the extremely rusty and stuck pistons...very messy but worked very well and no drama. I got each one to move then jammed with a piece of wood or G Clamp and then got the next to move until all three pistons were free. Then drove them all out more or less together. The grease gun pipe fitting was a perfect fit for the caliper brake pipe thread.
Last edited by paddyx350; Nov 3, 2021 at 05:10 PM.
Front Caliper Refurbishment
