Vacuum Brake Booster: Service and Repair
Removing And Installing Brake Booster
Removal
NOTE:
- To remove and install brake booster -C-, the brake master cylinder -B-, the hydraulic unit -D- and the booster pump -A- in vehicles with PSM (GT2 without PSM) must be removed.
- The assembly of the brake master cylinder -B- with the brake booster -C- must not be disconnected for removal and installation if both parts should continue to be used.
1. Switch off ignition. Remove cover -A and B-.
2. Drain brake fluid explansion tank. To do so, suck brake fluid out of the expansion tank or pump out the brake fluid using the brake pedal via a front and a rear brake caliper (use collecting bottle).
3. Disconnect the electrical line (plug) on the reservoir.
4. Undo the brake lines on the brake master cylinder. Close the brake lines and the brake line connections with suitable plugs. Undo the fastening nuts -A- of the brake master cylinder. Pull out brake master cylinder.
- The brake master cylinder must be disconnected from the brake booster if both parts should continue to be used.
-A- = Brake master cylinder fastening nuts
-B- = Brake booster fastening screws (Torx)
5. In vehicles with PSM, remove the booster pump -1- with the holder -1a-. For the removal of the booster pump, carry out the following removal steps:
- Remove suction line -2- for booster pump. Undo pressure pipe on the booster pump -3-. Close the lines and the pump connections with suitable plugs.
- Release plug -4- -arrows- and disconnect. Remove connector housing rearwards from out of the holder -moving arrow-.
- Undo the three plastic fastening nuts -5- and remove the booster pump -1- with the holder -1a-.
NOTE:
- In the case of assembly work on the hydraulic connections, the electrical plug connections on the hydraulic unit must always be connected.
- This prevents the plug connections on the hydraulic unit or in the plugs inside from being splashed with brake fluid or water.
- Any damp plug must be wiped dry before being disconnected from the plug on the hydraulic unit.
- Brake fluid on electrical contacts (even thinned brake fluid) leads to corrosion and may cause faults in the system.
6. Remove hydraulic unit. To remove the hydraulic unit, carry out the following removal procedure:
- Undo the brake lines on the hydraulic unit.
- Close the brake lines and the brake line connections with suitable plugs.
- Pull the plug off the pressure sensor if present (vehicles with PSM 5.7).
- Unlock/disconnect the control module connector on the hydraulic unit. Cover the plug connection and the plug.
- Undo and guide out the hydraulic unit.
7. Remove holder of hydraulic unit (if still installed).
8. Separate the 2 push rods from each other (loosen). For this purpose, remove the fastening nut (fastening nut, standard as of April 2001) -arrow-. To make the retainer or the fastening nut accessible, adjust the bellows positioned above the body.
9. Disengage the vacuum line on the brake booster. Undo the fastening screws -arrows- of the brake booster (Torx T 45). Pull out brake booster.
Installation
- When replacing the brake booster, observe the correct allocation according to the Spare Parts catalogue. Different transmission ratio (boost ratio) of the 911 Turbo (996) compared to the 911 GT2.
- Boost ratio 911 GT2 = 3.15 : 1. Boost ratio 911 Turbo (996) = 3.85 : 1.
1. Install new pushrod (push rod with fastening nut) -12.1- (if not already present) in place of -12- (push rod with retainer). When doing so, observe installation dimension X (138 mm).
Basic setting of the push rod -dimension X- is 138 mm (set in new brake boosters). If necessary, correct during the relevant work.
2. Insert brake booster with bellows -B- and tightened clamps -6-.
NOTE: Do not trap wiring harness.
3. Tighten fastening screws (Torx T 45) -arrow- to 23 Nm (17 ftlb.).
4. Connect the 2 push rods. To do so, mount the fastening nuts -arrow-.
NOTE:
- Adjusting the brake pushrod length changes the position of the brake pedal.
- Perform a fine adjustment of the brake pushrod if the correct position of the brake pedal is not given.
- After the brake pushrod is adjusted, check the brake light switch adjustment.
5. Check the position of the brake pedal -Dimension X = 45 + 5 mm-. Perform measurement between the centre of the brake pedal plate and the accelerator pedal. Perform a fine adjustment of the brake pushrod if the correct position of the brake pedal is not given.
- Fine adjustment of brake pushrods: Undo the lock nut for this purpose and the turn the push rod accordingly at the hexagon -arrow- until -dimension X = 45 + 5 mm- has been achieved. Subsequently retighten the the lock nut.
6. Fit the bellows onto the body and fasten with the clamps.
7. Check the setting of the brake light switch -1-.
The pedal must move a perceptible distance of approx. 5 to 10 mm before the brake light lights up. At the same time, check (make sure) that the brake light lights up before the pedal meets perceptible resistance or before brake pressure builds up. If necessary, set/re-adjust.
8. Fit the vacuum hose for the brake booster.
9. Install hydraulic unit, boost pump (in vehicles with PSM) and brake master cylinder. Fit all brake lines.
NOTE: Ensure that the reservoir is engaged correctly on the brake master cylinder.
10. Bleed the brakes Serv. No. 47 01 07 (vehicles with PSM) or bleed the brakes - G12 (without PSM) Serv. No. 41 01 07.
NOTE:
- First bleed brakes as usual
- Only afterwards (after the conventional bleeding) should the booster pump control be performed (continue bleeding) with the Porsche System Tester 2 in vehicles with PSM. If this is not observed, damage can occur to the booster pump.
- To bleed at front on right with running booster pump (PSM), the bleeding device should remain connected (switched on). Bleeding pressure approximately 2.0 bar.
11. When replacing the hydraulic unit in vehicles with PSM (4-channel system), code the control module (manual transmission or Tiptronic).
12. Carry out the following tests:
- Check the system (brake lines and connections) for leaks.
- Read out and erase the fault memory of the ABS or PSM control module with the Porsche System Tester 2 (PST 2). Additionally, perform the static test in the system.
- Check electrical and hydraulic lines (brake lines) for incorrect assignment using the PST 2.
NOTE: The exchange test checks that the brake lines/solenoid valves are assigned to the correct brake calipers (wheels).
- Then start the engine. The ABS and if necessary also the PSM-related lights in the instrument cluster must go out when the engine is running.
- Carry out a function check of the brakes. Then perform a short test drive and perform at least one ABS control operation, taking the road conditions into consideration. Read out the ABS or PSM fault memory again. There must not be any stored faults.
13. Install covers -A/B-.
