Brake Booster Clearance?

[tbryanh]

Brake Booster Clearance?

Does anyone know what amount of clearance is needed between the brake booster and the master cylinder?

To be more specific, the booster has a rod that pushes on the master cylinder when the brakes are applied.

The length of the rod is adjustable, and when the brakes are not being applied, the rod should not be touching the master cylinder. There is a clearance spec for this that I need to know.

I am changing both the brake booster and the master cylinder.

Thanks.

 

[spartacus]

Brake Booster Clearance?

Does anyone know what amount of clearance is needed between the brake booster and the master cylinder?

To be more specific, the booster has a rod that pushes on the master cylinder when the brakes are applied.

The length of the rod is adjustable, and when the brakes are not being applied, the rod should not be touching the master cylinder. There is a clearance spec for this that I need to know.

I am changing both the brake booster and the master cylinder.

Thanks.

The rod should be touching the master cylinder barely. I just used my old one to match the new.
Brake Booster install

 

[tbryanh]

Barely touching might be a little close. I've since found information that 1mm might be about right. This is just an initial clearance to be used on the bench when assembling the booster to the master cylinder.

Once the booster/master cylinder assembly is installed and the brakes bled, brake pedal freeplay needs to be checked (along with other checks). The master cylinder may need to separated from the booster (while keeping the brake lines connected to the master cylinder) so that the booster pushrod can be adjusted for the proper pedal freeplay.

Here are the specs from the factory service manual.

Brake pedal height (from asphalt sheet)
158.1 - 168.1 mm (6.224 - 6.618 in.)
Brake pedal freeplay
1 - 6 mm (0.12 - 0.24 in.)
Brake pedal reserve distance at 490 N (50 kgf, 110.2 lbf)
More than 75 mm (More than 2.83 in.)

Here are links to a couple of good articles.

Brake Pedal Free Play Adjustment Procedure
Brake Pedal Free Play Adjustment
Rob and Dave's Aircooled Volkwagen Pages (for the shade tree mechanic)

Brake Pedal Free-Play and Pedal Reserve
Home | LBCC
Phil Krolick – Automotive

Here is a good youtube video.
Brake booster checks, operation and pushrod adjustments #1313
https://m.youtube.com/watch?v=rYYzPIGoxfY Andy Mechanic

Notes:
The brakes can lock up if there is no freeplay in the pedal. This may not happen right away. It may only happen after heavy brake use.

Pedal freeplay has to be checked by pushing the brake pedal by hand (not by foot). It is suttle and can be confused with take up distance.

Brake pedal freeplay is the amount of pedal movement before the pushrod touches the piston inside the master cylinder, before the piston starts to move.

Take up distance is the distance the piston inside the master cylinder bore has to move before the brake linings engage with the drums/rotors; that is, the distance the pedal travels (after freeplay travel) before braking action actually occurs.