Power booster piston relation to master cylinder

[Ralph]

Having problems dialing in my brakes on a 73 cuda with power front disc and rear drum brakes

So my question is the power booster has a piston that goes into the back of the master cylinder, that piston has a nose which is adjustable by thread to make it protrude more or less.

Some tell me there should be a 1/8" free clearance between the two and some say 1/4 " clearance.

Any advice ?

Thanks to all

ralph

 

[moparleo]

Only the short rod in the booster contacts the master cylinder. It should be adjusted to just contact the master cylinder. This will only affect pedal height. What is your brake problem ?

 

[Chryco Psycho]

You really do not want any play in there , but you do not want the pushrod depressing the piston at rest either as the fluid may not return into the master , you can use a crayon to see if the tip of the pushrod is touching

 

[Ralph]

Pedal is low - rod is about 1/4" to 3/8" away

Believe most of my problem is the proportioning valve. The front brakes bleed different from most cars I have had. The front brakes do not engage well and the back wheels lock up and skid.

Working most other things out, right components, new master cyclinder, brake lines and hoses.

Sounds like I am going to adjust that rod to 1/8" away or less.

Thanks Ralph

 

[moparleo]

Does the pedal pump up or does it stay low even when pumping the pedal.

 

[Ralph]

Does the pedal pump up or does it stay low even when pumping the pedal.

In general it stays low and seems to have a lot of play in it.

I am getting a lot of answers on the piston clearance so I measured it, it seems I have more than 1/4"clearance - some say 1/4", some say 1/8" an some say closer.

I am going to bench bleed new master cylinder and want to put it back correct.


Thanks for your help

Ralph

 

[Avalanche]

I have the exact same issue you are having. I did a lot of research and what I found is there was a lot of problems with rear tire lock up. What I am doing is to put a adjustable proportioning valve going to rear brakes after distrubtion block.
I am getting this done this week. Will let you know if it solves the problem

 

[Chryco Psycho]

any free play between the pushrod & piston in the rear of the master is just wasting travel .
You can also solve the rear lock up problem by running larger diameter truck wheel cyl in the rear , I should be able to locate the part # if you need it

 

[Ralph]

Ok got it,

I will eliminate the free play

Give me the part # in case I go that way.

Was thinking about the wildwood adjustable proportioning valve as well, my rears were looking up as well

Thanks to all

Ralph

 

[Ralph]

So had my proportioning valve replaced and the brakes seem to be working better.

Still have some brake fluid spitting up from the master cylinder, what would cause that.

Everything is new or rebuilt. Have new flexible stainless steel lines on the front connecting to the calipers - should they be changed or checked for obstruction.

I have something incompatible here or working incorrectly.

Thanks Ralph

I have something

 

[moparleo]

They should be fine unless you have pinched them with pliers or similar toll . If done too hard you can collapse the inner liner.

 

[national]

Im having the same problem,I think The rod is set pretty close but is there a rubber seal or does the boot go on there? I bought new master cylinder & it came with both a boot & lil thick seal,not sure which goes where,the directions say for manual brakes but my car & new master cylinder are for power disc front,drum rear..Originally I couldnt get brakes to come up good,went to floor,real sloppy,exchanged for another new master cylinder,gonna give it a try,Im bench bleeding also,all brakes,calipers,hoses are new

They should be fine unless you have pinched them with pliers or similar toll . If done too hard you can collapse the inner liner.

 

[Chryco Psycho]

The rubber O ring / thick one is to lock the pushrod into the back of the master , but this is only used on Manual brakes not power . same with the boot .

 

[national]

Thank you so much

The rubber O ring / thick one is to lock the pushrod into the back of the master , but this is only used on Manual brakes not power . same with the boot .