Brake Bleeding Issues on Hybrid Brake System

[CBDZ]

Hello all,

I've been fighting my custom brake set-up for over a week now. What I have is a stock, dual chamber master cylinder, stock front wheel cylinders on the front, with disk brake calipers from a Datsan (Nissan) 280zx on the rear.

I have bled the system about 5 times, using every type of method that I can think of. I just can't get the rears to build pressure, and therefore I have zero rear brake movement.

This is similar to the conversion done here, but I am using a dual circuit M.C. while he uses a single.

http://blindchickenracing.com/How_to/28 ... brakes.htm

I have thought that maybe I need residual pressure valves, but maybe not because the m.c. and reservoir is higher than the calipers and wheel cylinders.

Any insight that you could share with me would be greatly appreciated.

[Ol'fogasaurus]

Three questions in no particular order:

What kind of problems are you having?

How have you been bleeding your system?

Did you replace the soft lines?

Lee

[CBDZ]

Lee,

All the flex lines are new. In fact, the only components that aren't new are the calipers from the 280zx.

I have "bled" the brakes every way that I can possibly imagine. I have done the two person, open-push, close-release method, pressure feed fiuld from the master cylinder side, pressure feed from the caliper side, gravity feed from the M.C. to the caliper. The only thing that I haven't done is to fully suck-down the system as you would an air conditioning unit.

What happens, is that I bleed and bleed and then bleed the system again. Every time it appears that there is no air in the system, but I am not getting any pressure upon closing the bleeder valves, and zero braking action from the single piston calipers.

[buildabiggerboxer]

Suspect m/cylinder rear section. I'm assuming the front is good, you don't say, Clamp off the rear flexis, the pedal should go hard, if not, have a look at the master, if it's old, it's likely gone awol because when pump bleeding you are pushing the seals into a corroded bore, if it's new, then the callipers need close scrutiny for free piston travel, but it may still be a bad master.

[Ol'fogasaurus]

CBDZ: one of the things I have found when working with VWs is to adjust the brake shoes (the front in this case)to a lock position then bleed the system then readjust the brakes properly. This is to center the semi-free floating brake shoes. As buildabiggerboxer suggested, and assuming that the MC is OK, one or both of the calipers is on the fritz otherwise you shouldn't have a problem.

Two other possible things: The MC is sized wrong or there is an air leak in the tube joins which is why the question on soft lines.

You are correct in talking about the normal requirements of installing/the use of residual pressure valves but I have heard a couple of times that by installing them did solve problems.

The last thing could be you need to have a proportioning valve as the two systems (front and rear) are too far apart size wise. An example could be that the rear brakes are coming in too soon or the opposite; they need to be balanced to where you want full braking to come in. There are adjustable valves out there.

[Steve Arndt]

I've had better luck with vacuum bleeders than pressure bleeders. Less mess too. Depending on the orientation of your calipers and the bleed fitting you may need to unbolt and raise the calipers up high so the bleed fitting is at the highest point.

On my street car I have a dual circuit standard master, ghia style front discs, CB rear discs, and a 2 pound RPV on the rear circuit. It took quite a while to suck all the air out. I also tried pedal and pressure bleeding first.

[Ol'fogasaurus]

I haven't had good luck with using vacuum bleeding on VWs: if you adjust all four wheels until they are locked up then vacuum bled the lines then maybe. It always seems to be the shoes not adjusted correctly that are the culprit.

Gravity bleeding also works if you know how to do it properly... assuming the brake shoes are properly adjusted.

I still have had the best luck with the old "buddy in the driver's seat" pumping the brake pedal for me: adjust up the brake shoes until they are solid locked and the wheels (with shoes/drum brakes) won't turn at all then, starting with the brake farthest away from the MC: pump the brake pedal until "hard pedal", "hold it", bleed, "floor", lock, repeat until no bubbles and the dark fluid are both gone then move on to the next wheel making sure the MC reservoir is kept full.

Lee

[CBDZ]

Problem solved. The way I have the calipers mounted put the bleeder valve at a low spot on the caliper. I simply dismounted the caliper from the trailing arm, so that the bleeder valve was at the top completely took out the bleeder valve, and let the fluid flow for a few seconds. It was quickly evident that this was a problem area as there was quite a bit of air that burped out.

I've got pressure, and working brakes.

Thanks again guys for the tips.

[Ol'fogasaurus]

Way to keep with it... good job! Forgot about that problem as it has come up before but not recently.

[Steve Arndt]

Glad we could help. Removing the calipers to get the bleeder up on top is pretty standard procedure.