Got my XJ12 back today after a diff rebuild and rear brake job. The car now vibrates from the front end when braking (warped rotor?) and the brake pedal goes soft after a while of constant pressure then needs to be pumped a couple of times to return to normal. Also when going down a steep hill there's an odd chirrup ... sounds like hamsters .... any ideas?

 

Well when the pedal goes soft does it maintain its braking force. If not it sounds like it would be air in your brake lines and they'd need to be bled. Not a difficult task. The reason your pedal would go soft is because hydraulic fluid (brake fluid) doesnt compress, but air does. So if you had air in the system it would compress causing lack of braking power and a spongy pedal.

Check for a warped rotor as far as vibration. It will look like it has uneven wear marks.

The chirps could be your brake pads either being worn in or overly worn. You said your rear brakes were done so they may be bedding in, or you might be on the last legs with your front brakes.

Let us know what you discover

 

looks like the master .... gone after a flush which apparently happens... right on the warped rotors.... hamsters are bedding in methinks...,. What a wonderful car to drive though.... I am in love !!

 

If you hear the noise only when the incline changes, I would suspect a mount. Check the V-mounts that support the rear cage. Perhaps they did not change them when diff was rebuilt or did not tighten them properly and it's allowing the driveline to shift.