ive removed the brake cap and removed the right rear tire and rim. I'm putting on new brakes and rotors on my 2001 jag s type. Normally to collapse the brake piston ive always removed the brake cap. and with a c clamp you can normally move the brake piston back into the bore so that you can put on new pads. The brake resovoir fills back up.. I started at the rear first .. Does Jag have some kind of check valve , Do i have to open the bleeder valve to get the piston to move ?? Confused , help please !!!

 

First- put the cap back on. You're letting airborne moisture contaminate the fluid. There's no need to remove of loosen the cap or the bleeder.

Next- the rear calipers also serve as the parking brake mechanism. The pistons screw back in, not push in as they would on a front caliper. There's two slots on the end of the piston. Make or purchase a tool that will engine the slots to allow you to turn the piston.

This is all detailed in JTIS.

 

makes perfect since thank you s much !!

 

An angle grinder collar tool, the type with the 2 pins, also work but they're a bit harder to use as they don't push on the piston like the correct brake piston wind-back tool does.

Clockwise is the direction to retract the piston.

 

Any Advance Auto Parts store will loan you their brake pads replacement kit which contains the necessary tool to properly rotate and compress the S-Type's rear caliper pistons into the caliper so you can complete your rear brake job. They'll require a deposit from you at tool kit pick-up, but your deposit will be refunded in full once you return their kit....

Be aware that only the rear brakes on these cars require this kit. The front brake pistons can be compressed with a regular C-clamp....

 

Sometimes, if the correct tool is not available, I am able to use a pair of needle-nose pliers to get the job done. Just open them so the ends are in the holes in the caliper piston and push in while turning clockwise.

 

Brake fluid is hygroscopic. It absorbs moisture from the air. It will do this over time with the cap on or off. When changing brake pads, it is a very good idea to NOT push the old fluid back into the system. Any contamination, rust or dirt will potentially be pushed into the master cylinder and possibly damaging the rubber seals causing leakage and brake failure.

I put a small hose over the bleeder screw and put the other end in a jar to collect the oil you push out. The when I'm done I fill the reservoir with fresh fluid.

 

Modern cars (1970s onwards) have a diaphragm that separates the air from the fluid in the master cylinder. There is no direct exposure when the cap is installed.

 

All caps are vented so the fluid is still exposed to air and it still absorbs moisture. I flush about every 3-4 years and it is needed!
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The vent is on the 'atmosphere' side of the diaphragm and allows air to enter as the fluid level drops due to pad wear. The fluid itself is on the other side of the diaphragm, not exposed to air.

That's why it's there.

 

There are seals. None work 100%. Gradually moisture gets past them. At the caliper (piston), at the reservoir. Others?

 

Agreed. No brake fluid system is hermetically sealed. But some do better than others when it comes to minimizing moisture....