Another brake booster pushrod question...

[French demon]

Hello guys

I recently replaced both the master cylinder and brake booster.
A brand new master cylinder, and a used (but recent) booster. It's an aftermarket unit from Summit racing, initially for E-bodies and the bracket wasn't suitable.
The new booster studs are bigger, so after having enlarged the original bracket holes, I've been able to install it.
Ok.

Now, my Demon brakes efficiently and without effort.
But... the pedal goes almost to the floor before having braking.
Is it due to a too much short booster front pushrod? The front pushrod is adjustable, but it's now at its maximum length.
And is the rear rod (between the booster and the pedal linkage) adjustable?

What can I do? All help will be appreciated.
Thank you
Francois

 

[Sdriche]

Did you use the tool to measure the adjustable booster rod length. It looks like an 'H" with a rod in the middle. If you have the booster rod too short you aren't getting engagement until the pedal is partially depressed. If you see a gap and the pushrod is fully extended it's too short.

 

[French demon]

Unfortunately, I do not have this kind of tool. I also suspect my pushrod to be too short.
Tomorrow, I'll try to compare this pushrod with the pushrod of my old original booster. Maybe they are interchangeable.

 

[AJ/FormS]

If you don't find it in the pushrod,
recall that the frontmost reservoir gets plumbed to the rear brakes. Inside the M/C, the front brakes do not begin to apply until the rear shoes are pushed against the drums, which then provides a hydraulic resistance to the powerpiston, and further piston travel forces the fluid to the front brakes.
Excessive pedal travel will then happen if both of the following conditions exist;
1) the pushrod length is too long, and either
2a) the power chamber has air in it, or
2b) there is excessive brake shoe to drum clearance at the back,
What happens is that the front power piston, for lack of hydraulic resistance, slides all the way to the front of the M/C bore, chased by the rear piston.Nothing happens until the front piston either hits the front of the bore or hydraulic pressure builds up between the pistons. If simultaneously the pushrod is too long, then the C-ports do Not open, and so if there is air trapped in the front of the front piston,it ain't ever coming out.

 

[Sdriche]

does the pedal return to normal height after releasing? The return spring is in the MC so that may help you figure this out. Bottom line, you have to adjust the push rod correctly. You can even make a tool from wood and a bolt shaft

 

[BillGrissom]

I have the plastic rod measurement tool of post 5 ($15 ebay I recall). I tried vernier calipers, but hard to get a consistent measurement. Most boosters have an adjustable output rod. It is critical that you match that to the MC. If too short, you will have lost motion. If too long, the MC piston can't fully retract to expose the inlet port which can cause the brakes to lockup when driving.

The adjustment rod you speak of at the pedal attachment simply affects where the pedal ends up sitting (for convenience). Probably best if at the same location as the accelerator pedal if you use the recommended 1-foot driving, to not snag your foot when moving between pedals. I find myself using both 1-foot and 2-feet since a long time since I had a stick-shift. Ensure that the brake lamp switch doesn't prevent motion of the pedal. It should not bottom out on that switch.

 

[crewchief]

I'm experiencing the same issue for years as if my power brakes aren't there. Rears are correctly adjusted and the pushrod was set using the abovementioned tool. The new master bled several times and Dewey rebuilt the booster.
Still at a loss!

 

[Ironracer]

I have the plastic rod measurement tool of post 5 ($15 ebay I recall). I tried vernier calipers, but hard to get a consistent measurement. Most boosters have an adjustable output rod. It is critical that you match that to the MC. If too short, you will have lost motion. If too long, the MC piston can't fully retract to expose the inlet port which can cause the brakes to lockup when driving.

The adjustment rod you speak of at the pedal attachment simply affects where the pedal ends up sitting (for convenience). Probably best if at the same location as the accelerator pedal if you use the recommended 1-foot driving, to not snag your foot when moving between pedals. I find myself using both 1-foot and 2-feet since a long time since I had a stick-shift. Ensure that the brake lamp switch doesn't prevent motion of the pedal. It should not bottom out on that switch.

2 feet here, no matter what I'm rolling! Just faster, more efficient, and I Belive, Safer