hunterpeaks

I switched out a leaking ABS pump on my 964 and can't get brake fluid to my rear brakes. Front brakes are good. Does anyone have the step-by- step method of priming this thing?
I read that it needed to be primed BEFORE I install it? If so, Crap! And then how do you prime it? Use a Turkey baster to squirt brake fluid in there? Tricks?

And then I have read conflicting reports on if I need to trick the PDAS on by wiring it up since I don't have the "hammer" tool? Can anyone confirm or deny this? And if I need to do this, then how?

Thanks.
Hunter

Spyerx

Need to run the pump while bleeding. Ie, pressure bleeder with pressure, open valves on brake, run pump, cycle locks, close valve. Do that until no air.
However, you should be able to get fluid to flow... so that's odd its not.
In my case, there was air in the pump that I replaced. (I had a lock frozen)

I used the ABS Test Set (sold here) to do so. But, basically run the pump, activate the locks. This could be done by jumping relays on the pump (likely) I have a C2...

You sure you don't have a blockage somewhere in the rear circuit?

hunterpeaks

Thanks. yes, Pump is on when bleeding. Valves open on one rear brake, and pumping the brake pedal 100+ times while pump is cycling. No fluid coming to back.
So, I am Not activating the locks. That's the jumping/wiring on the relay plugs, you are referring too?

No blockage that I know of. Have been working like normal. But, the old ABS pump starting leaking and I lost all the fluid in the frunk. Very scary going into turn 1 at Sebring and brake goes to the floor!!! Fortunately, after 4 quick pumps it came back and I came in the next lap to a Frunk full of fluid.

Hunter

Spyerx

are you using a pressure or vacuum bleeder ? If so you shouldn't need to pump pedal.

hunterpeaks

Neither. Using my son to pump the pedal. I have a pressure bleeder but since he's free labor....

Spyerx

try that to get the flow going. Motive pumped up to 15lbs or so, open 1 rear bleeder, don't pump the pedal. see if you get flow.

-nick

Brand new abs unit? The path from master cylinder to caliper is wide open, unless an abs solenoid is stuck in the “pressure holding” or “releasing” positions.

The rear brake bias valve is installed in the correct direction? It’s right off of the rear axle abs unit line.

I second not using the pedal. That’s a sure path to master cylinder seal destruction. Also, don’t use the pedal when you pressurize the brake reservoir with a bleeder. That can blow the reservoir lines off.

cjoenck

Actually pumping the pedal can seriously damage the master cylinder - at least on the C2 - so I never do that. The pressure bleeders are not that expensive. Certainly cheaper than a new master cylinder.

bw-porsche

Have a look at the attached. It applies if you have replaced the pressure switch on the hydraulic brake pressure pump. Hope it helps!

hunterpeaks

Thanks for everyone's help. It turns out that I created the problem. In hopes of creating a more even brake bias from front to rear, I "gutted" the brake bias pressure valve. Gutting it prevented any fluid moving to the rear brakes! So, I put all the guts back and instantly fixed the problem. Have fluid now in the back brakes again! Except now one of the lines coming from the firewall to the ABS pump is leaking. UGH. To replace that now.

Goughary

Can i ask- how, if you took out the internals, was the line then blocked? What was in there blocking the line to the rear?

hunterpeaks

The spring that I took out is what causes the valve to open, along with a washer in there. So, I "think" just replacing the spring and washer would have fixed it. But, I also replaced the "ring" as well.

-nick

Ah, that would do it! The spring in the bias valve normally keeps the piston/plunger away from its seat and the fluid in the line is free-flowing to the rear calipers. When fluid pressure from the master cylinder is high enough to overcome the spring, the plunger seats and blocks additional pressure to the rear calipers. I'm not sure how gradual the pressure is reduced in our stock valves, or if it acts more like an on/off switch.

The valve is very simple and clever, and can be a performance advantage when tuned for the particular dynamics of the situation. In most braking circumstances, the valve does nothing. Only under heavy braking does the valve reduce rear brake torque in an effort to compensate for forward weight transfer.

What is your motivation for gutting the valve? Too much rear brake torque can quickly lead to unstable handling.

Here's what my '91 C4 valve looks like. Master cylinder pressure comes in from the left (via the abs), output to the rear calipers through the elbow adapter on the right. The plunger is pushed to the right under high pressure and seals the exit hole.