Can't get brake pedal pressure

[paul mc]

Hi Guys,

As I do not use my 1987 RRC enough any more I noticed that last time i took it out on the road the brakes were binding, i assumed because the calipers had seized slightly through lack of use so while it was braking very well, the pistons were not retracting enough to avoid heat build up and causing the brakes to rub and overheat. So, a couple of weeks ago I took off each caliper one by one and gave the exposed bit of piston a little clean and then pushed them back home fully to try to avoid it binding again. The front drivers side caliper was a little worse so i took the pistons out and gave the spots of light rust a light rub before replacing and again pushing the caliper all the way home. When it was all back together and bled the drivers caliper (which is the only one i disconnected from the hoses) i have absolutely no brake pressure at all. Brakes have always been fantastic on this prior to now.

What i have done since

 - complete brake bleed in correct order of primary and secondary circuit, both one man bleed kit and pressurized easibleed.

 - fitted new master cylinder as the old reservoir had a split it the top anyway, primed the master  cylinder off the vehicle

 - full brake bleed again until no bubbles coming through anywhere, again bled with one man kit pumping the pedal and with easibleed. I've used 4 litres of fluid so well and truly bled!

What have i missed? the pedal goes completely to the floor even after all of the bleeding, could i have damaged something by forcing the pistons back into the calipers without opening bleed nipples, i.e. I forced the fluid back into the master cylinder but i do not see what this would damge as the pedal was not depressed so should allow fluid to flow backwards? No leaks anywhere so where in the brake circuits can fluid recirculate past a seal, as the master cylinder is pumping fluid and the bleed process worked on both circuits. Fluid appears to be coming from all bleed nipples in good quantities. Pedal is fairly stiff with engine off so servo is working well.

If i clamp one of the front flexi braided lines on the secondary circuit, the brake pedal stops just short of full travel, no difference if i clamp one of the front primary circuit but not sure what this means.

I literally do not know what to do next as I can not understand where the problem might be and just keeping bleeding is not working.

Any help very much appreciated.

Thanks, Paul

 

Edited October 15, 2017 by paul mc

[paul mc]

Just a a quick update to see if it helps. I thought i would try disconnecting the brake lines from the master cylinder and blocking the two outlet ports on the master cylinder with a bolt each. The brake pedal was then completely solid which would suggest the master cylinder is fine, but if i only block one of the ports and replace the brake lines on the other one the pedal is rubbish again, this is the same regardless of if i block the front or the back ports.

Is this a good test of the master cylinder or by blocking both ports would this cause a solid pedal even if the master cylinder seals were knackered?

[Maverik]

Blocking both ports would suggest to me that your master cylinder is ok, but depending on what model cylinder it is, the way they work is via 2x pistons on the same spindle with like a compensating spring between the two, its designed that if one of the piston seals goes i.e. the front one then the rear piston will partial activate the front brakes as well as the rear but with greatly reduced performance...

I does just sound like you've just got air trapped in the front and rear circuits still.

What kind of valve is in the middle of the system? a G valve, proportional valve or compensator valve? Anywho I've found that some brake systems seem to take a bit more effort to bleed than others, usually ones with a compensator valve and often some pretty aggressive pumping of the pedal is needed to shift difficult air bubbles - I mean hard fast pedal action, best done with a self bleed kit with a non return valve on the nipple end... even if your master cylinder is bypassing, you should be able to get a hard pumped up pedal but will then slowly go to the floor...

Also try the pump up the pedal, get it to some resemblance of hard then open the bleed nipple, you get a "whoosh" of fluid also helps move stubborn bubble problems...