When working on the booster circuit for removal and installation of corresponding parts first depressurize this circuit

To do this, disconnect the electrical plug on the pressure pump (pump assembly) (refer to drawing 348-47) and then operate the brake pedal approx. 25 times. To remove and plug in the electrical plug, press in the center on the plug locking clip.

Preparatory measures for first assembly or reinstallation

This work step (1 st step) is required only in the event of replacement or re-installation of the pump assembly, pressure accumulator and valve block. In addition, it is also necessary if the suction line has been opened or if the fluid reservoir was previously empty. Otherwise, start with the 2nd step. (Bleeding the pressure accumulator and the valve block).

Ensure that the electrical plug at the pressure pump (pump assembly) has been disconnected.

348-47

Immediately after assembly of the parts, fill the fluid reservoir with new brake fluid up to the top edge. Connect the bleeding device to the fluid reservoir. Clamp off the overflow hose (ventilation) with a hose clamp. Switch on the bleeding device. Bleeding pressure approx. 1.5 bar.

911 Carrera 4 Brakes, Hydraulics, Regulator, Booster

Note

The brake pedal must not be operated as long as the bleeding device is connected, otherwise the return line may be forced out of the rubber plug of the brake booster.

Continue with bleeding the pressure accumulator and valve block.

Move the steering/ignition-starter lock to position 1 (necessary for pump operation). Plug the electrical plug onto the pump. As soon as no air bubbles are visible any more at the transparent bleeder line of the collection bottle, disconnect the electrical plug and close the bleeder valve.

Bleeding the pressure accumulator and valve block (step 2)

Note

Insofar as the filling and bleeding device is not connected (is required only for step 1), always check the fluid level in the fluid reservoir between the bleeding operations and top up with new brake fluid if necessary.

If step 1 was not performed, discharge any accumulator pressure. To do this, disconnect the electrical plug at the pressure pump (pump assembly) and slowly open the bleeder valve at the pressure accumulator with connected collection bottle. Hold the bleeding hose firmly.

Caution:

A pressure of up to 180 bar exists at the bleeder nipple of the solenoid valve and also at that of the pressure accumulator. Open the bleeder valve only very carefully. Ensure that the bleeder hose is securely fitted! Wear protective goggles and protective gloves.

Open the bleeder valve at the valve block.

Plug the electrical plug onto the pump. Disconnect the electrical plug and close the bleeder valve as soon as the brake fluid emerges without air bubbles.

Now completely fill the pressure accumulator (bleeder valves closed). For this purpose, connect the electrical plug. As soon as the pump has audibly switched off, disconnect the electrical plug and completely discharge the pressure at the bleeder valve of the pressure accumulator. Slowly open the bleeder valve and hold the bleeder hose tightly. Caution: A pressure of up to 180 bar is present.

Wear protective goggles and protective gloves.

If not already done (if the system was already depressurized), connect the collec* tion bottle to the pressure accumulator bleeder valve and open the valve.

Repeat the last operation (completely fill the pressure accumulator and then completely drain the accumulator pressure) approx.. 1-2 times (brake fluid must be free of air bubbles).

Also bleed the valve block in the same way (as for the pressure accumulator) approx. 1-2 times.

When it is guaranteed that bleeding has been performed so that there are no longer any air bubbles, securely tighten the bleeder valves and plug the electrical plug onto the pump. Ensure that the plug engages properly.

Switch off the bleeding device if appropriate and disconnect. Also remove the hose clamp from the overflow hose (ventilation).

Press the brake pedal several times. (The bleeding device must not be connected.)

Continue with bleeding the lock cylinders and pressure lines.

Bleeding the lock cylinders and pressure lines (step 3)

In the following work step, the solenoid valves are operated with the system tester 9288. The high pressure present at the solenoid valves is thus supplied in pulse form into the pressure lines and to the lock cylinders via the check valves.

Remove brake fluid at the bleeder valves of the lock cylinders until no air bubbles appear anymore. (Top up fluid reservoir in between if necessary).

The exact work sequence Is described on Page 47-5.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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