bleeding uprated brakes...

[Nickpascoe]

Help,

I've just fitted rimmerbros 4 pot calipers to a TR7 and try as i might i just cannot get the system to bleed. I've 20 years tinkering with cars and know how to bleed brakes. What happens is that even 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 pistson are moving freely and there is no excessive movement or retraction 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 a tip top condition if it is to pump enough fluid into them to clamp the pads onto the disks. Problem is though, the car was braking well enough before i fitted the kit a few days ago.

Ideas anyone?

Nick

[Raider]

Could the calipers be fitted to the wrong side?

IE offside to nearside and vice cersa?

If so the bleed nipple will be in thew wrong place and you can't get all the air out.

I only ask because I did this with Volvo 4 pot calipers  ::)

[Nickpascoe]

Thanks but only one viable way to fit these calipers. 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, although i'm suspecting the master cylinder seals. The first push of the pedal goes to the floor and the 2nd, 3rd and 4th build to a good firm pedal, although iwth a bit more travel than ideal. I'm concluding, therefore, that there cannot be air in the system as the pedal would never be firm and that one of the two master cylinder chambers must be leaking into the other. This way, the master cylinder would only pump a smallish slug of fluid (hence the first pusj goes to the floor) and the subseqeunt pushes would push a bit more fluid in building the pedal to a firm position. Changing the master cyl;inder seals next week to check if this is the cure.


Nick.

[Raider]

OK - point one.  Why not use a Gunson's easybleed. This does away with the need to pump the pedal at all and means a constant, stady pressure through the system.

Point 4 - yes I have heard this and have to confess that with my brake set up the pedal isn't exactly firm. It is better than it was tho. It passed it's MOT ok but about two weeks later I put another half litre of brake fluid through the front calipers using my Easybleed and, sure enough, there was some air still in there.

It was much better afterwards.

[Nickpascoe]

You don't work for Gunsons do you? I use a bleeding bottle bought from Demon Tweks that has non-return valves in it, so in theory, bleeding is a one man job, although i still prefer to do it the one man one bored wife way.

[Raider]

Hah - no I don't ;D

An unsolicited testimonial.

Good luck with those bleedin' brakes :o