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MG MGA - Bleeding uprated TR7 brakes

Help,

I've just fitted 4 pot calipers to a TR7 and try as i might i just cannot get the system to bleed. I've tinkering with cars for 20 years and know how to bleed brakes. What happens is that after lot of bleeding, the first full push of the pedal goes to the floor and the subsequent ones build a little pressure. Pause a bit and the pedal goes to the floor again. I've checked all 8 pistons are moving freely and there is no excessive movement of the pads (taking all the moving fluid). I'm down to thinking that it must be the brake master cylinder seals and that as the 4 pot calipers have a greater moving volume of fluid, the master cylinder must be working in tip top condition if it is to pump enough fluid into them to clamp the pads onto the disks.

The bits of advice i've had thus far:

1. try altering the rythem of pumping when bleeding the brakes to disturb any slug of trapped air. eg try 6 very fast pumps then a few slow ones with the nipple open (and connected to a tube into a jar with fliud in etc)
2. check the rear cylinders have been connected correctly in that the inlet brake pipe for each should be the lower of the two ports and the outlet / bleed nipple should be the higher port
3. when bleeding, there is a chance that you may damaage the internal master cylinder seals as during normal driving, you only use a limited range of brake pedal / master cylinder travel. During bleeding, you push the brake pedal / master cylinder to the floor and the seals move away from their shiny bedded in part of the master cylinder bore onto a part that is less polished (and may contain dirt).
4. the bigger brake kit should be fitted alongside a larger master cylinder and servo that moves a larger slug of fluid if the brake pedal travel is to remain sensibly short.

No definite solution yet though.

Nick.

PS. why is it that whenever i ring RimmerBros i get the feeling that i'm bothering them, miserable humourless bunch that they are.
nick

Most 4 pot calipers have more than one bleed nipple on them and must be done correctly to ensure the air is completely out of the cylinder. Also have you mounted them on the correct sides, They are handed and this can be seen by having a bleed nipple uppermost on the cylinder.
Bob (robert) Used to have 2 cats

Do you have a tandem master cylinder? If so, has the shuttle valve in the front/rear pipe junction moved to one end?

Mick
Mick Anderson

This thread was discussed between 14/10/2005 and 15/10/2005

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