Checking brake booster - AR43.10-P-0325RT

MODEL 251

Fig 1: Checking Brake Booster

Courtesy of MERCEDES-BENZ USA

	Checking
	Risk of poisoning caused by swallowing brake fluid. Risk of injury caused by brake fluid coming into contact with skin and eyes.	Only pour brake fluid into suitable and appropriately marked containers. Wear protective clothing and eye protection when handling brake fluid.	AS42.50-Z-0001-01A
	Risk of death caused by vehicle slipping or toppling off of the lifting platform.	Align vehicle between columns of vehicle lift and position four support plates at vehicle lift support points specified by vehicle manufacturer.	AS00.00-Z-0010-01A
	Brake fluid notes		AH42.50-P-0001-01A
	Notes on repairs to brake system		AH42.00-P-0003-01A
1	Open engine hood and raise to vertical position		AR88.40-P-1000RT
2	Connect vacuum tester (1) to brake booster (3).	Observe manufacturer's operating instructions in order to prevent erroneous measurement or faulty operation. Reduce vacuum of brake pedal beforehand by pressing pedal several times.	AR43.10-P-0024-01RT
		Test connection	Figure
		Vacuum tester	Figure
3	Connect brake fluid changing equipment	Pay attention to manufacturer's operating instructions. Brake fluid change unit
4	Raise vehicle using two-post lift	Only lift vehicle until wheels have sufficient ground clearance. Two-post lifting platform 3.0-3.5 t, electromechanical	AR00.60-P-1000RT
5	Take off left front wheel
	Remove/install wheels, rotate if necessary		AP40.10-P-4050RT
6	Remove bleed screw from floating brake caliper (7) and connect pressure tester (6).	Pressure testers for hydraulic oil systems must not be used for systems with brake fluid otherwise brake fluid is mixed with mineral oil which can cause the brakes to fail. Observe manufacturer's operating instructions in order to prevent erroneous measurement or faulty operation. Collect escaping brake fluid. Pressure tester
7	Bleed pressure tester (6) with brake fluid replacement unit	Observe manufacturer's operating instructions in order to prevent erroneous measurement or faulty operation. Brake fluid change unit Pressure tester
8	Switch off brake fluid changing unit and release pressure	Brake fluid change unit
9	Install pedal force meter	Observe manufacturer's operating instructions in order to prevent erroneous measurement or faulty operation. Install pedal force meter sensor (4) at brake pedal and set up pedal force meter display unit (5) so that it can be seen. Pedal force meter
	Risk of accident caused by vehicle starting off by itself when engine is running. Risk of injury caused by contusions and burns during starting procedure or when working near the engine as it is running	Secure vehicle to prevent it from moving by itself. Wear closed and snug-fitting work clothes. Do not touch hot or rotating parts.	AS00.00-Z-0005-01A
10	Start engine and create a vacuum of 0.75 to 0.8 bar by accelerating and then suddenly releasing the accelerator pedal.	If a significantly lower vacuum is reached or if the vacuum drops again immediately, the O-ring between the brake booster (3) and tandem master brake cylinder may be damaged or the vacuum line with check valve (2) may be defective: ↓
		Checking check valve	AR43.10-P-0023RT
		Remove/install stepped tandem brake master cylinder, install new O-ring.	AR42.10-P-0310RT
11	Apply five specified brake pedal forces to brake pedal in succession	Read off and note the relevant measured values of the brake line pressure on the low pressure gauge or on the high pressure gauge of the pressure tester (6). Measured values must be within specified ranges. With each measurement, ensure that a vacuum of 0.75 to 0.8 bar is maintained.
		Brake pipe line pressure	*BE43.10-P-1002-01E
		Test connection	Figure
		Vacuum tester Pedal force meter Pressure tester	Figure
12	Turn off engine
13	Remove pedal force meter	Pedal force meter
14	Detach the pressure tester (6) and screw the bleed screw into the floating brake caliper (7) again	Collect escaping brake fluid. Pressure tester
15	Bleed brake system	After the test only bleed the floating brake caliper (7) to which the pressure tester (6) was connected. Pay attention to manufacturer's operating instructions. Front axle:	AR42.10-P-0010RT
		Bleed screw to brake caliper Rear axle	*BA42.10-P-1001-10L
		Bleed screw to brake caliper	*BA42.10-P-1001-12L
		Brake fluid Brake fluid change unit	*BF42.10-P-1001-01H
16	Install left front wheel
	Remove/install wheels, rotate if necessary		AP40.10-P-4050RT
17	Lower vehicle using two-post lift	Two-post lifting platform 3.0-3.5 t, electromechanical	AR00.60-P-1000RT
18	Detach brake fluid replacement unit	Brake fluid change unit
19	Disconnect vacuum tester (1) from brake booster (3)		AR43.10-P-0024-01RT
		Test connection	Figure
		Vacuum tester	Figure
20	Close engine hood		AR88.40-P-1000RT

PNEUMATIC BRAKE BOOSTER TEST SPECIFICATIONS

Number	Designation					Model 251
BE43.10-P-1002-01E	Brake pipe line pressure	Intake manifold vacuum	Force on brake pedal	50 N	bar	26.5...32.5
		0.75...0.8 bar		100 N	bar	50...57
				150 N	bar	72...82
				200 N	bar	95...107
				250 N	bar	118...124

FRONT AXLE BRAKE CALIPER

Number	Designation		Model 251 except 251.077/177	Model 251.077/177
BA42.10-P-1001-10L	Bleed screw to brake caliper	Nm	18	18

REAR AXLE BRAKE CALIPER

Number	Designation		Model 251
BA42.10-P-1001-12L	Bleed screw to brake caliper	Nm	7

BRAKE FLUID

Number	Designation			Model 251
BF42.10-P-1001-01H	Brake fluid	Filling capacity	Liters	≈0, 8
		Specifications for Operating Fluids	Specification	DOT 4. See GENERAL BRAKE FLUID (https://bevo.mercedes-benz.com/index.php?language_id=1)
			Specification	MB 331.0 brake fluid A 000 989 08 07

Fig 2: Identifying Test Connection (168 589 08 63 00)

Courtesy of MERCEDES-BENZ USA

Fig 3: Identifying Vacuum Tester (201 589 13 21 00)

Courtesy of MERCEDES-BENZ USA