Brake drag and booster/MC rod adjustment

[weq92f]

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I've read quite a few threads on here today about dragging/locking front brakes. What I've gleaned from the discussions tells me a potential cause is the rod length which can restrict the MC piston from releasing 100%. I replaced the calipers and hoses because the dragging is a recent development after many miles on the installed components and figured the rod must be right.

Now they lock up and stay that way. So I released pressure at all points starting at caliper -> PV out(front) -> PV in(front) -> MC out front, one at a time each time locking the brakes back up with some pedal pump. Each point releases pressure to both front calipers. So...the problem is the MC...right?

One more test...lock brakes and unbolt the MC from the booster. That unlocks both calipers too.

Pedal rod into the booster or the booster pushrod need adjusting to reduce the MC preload. Can you guys help me understand how to do that?


-Kevin

[Sheepdip]

Originally Posted by weq92f

.

I've read quite a few threads on here today about dragging/locking front brakes. What I've gleaned from the discussions tells me a potential cause is the rod length which can restrict the MC piston from releasing 100%. I replaced the calipers and hoses because the dragging is a recent development after many miles on the installed components and figured the rod must be right.

Now they lock up and stay that way. So I released pressure at all points starting at caliper -> PV out(front) -> PV in(front) -> MC out front, one at a time each time locking the brakes back up with some pedal pump. Each point releases pressure to both front calipers. So...the problem is the MC...right?

One more test...lock brakes and unbolt the MC from the booster. That unlocks both calipers too.

Pedal rod into the booster or the booster pushrod need adjusting to reduce the MC preload. Can you guys help me understand how to do that?


-Kevin

See diagram on post #80..... http://www.67-72chevytrucks.com/vboa...61#post9306361

[RichardJ]

.>>lock brakes and unbolt the MC from the booster. That unlocks both calipers too.<<

Start by checking free-play on the brake pedal. Use 2 fingers to press down on the pedal for 1/8" to 1/4" of free play.

Measure the distance you have to move the MC flange away from the booster. Add a 1/16" to the distance measured and that is the distance you must shorten the booster pin.

They do have a tool to check the pin clearance.

You can take the measurements with a depth gauge, but you have to be able to move the MC a good distance away from the booster to get in there or remove the brake lines.

[weq92f]

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Let’s say I need to shorten the plung rod that goes into th MC by 1/4 inch. Where is that adjustment made and how?

I’ve read it’s made at the rod running from the brake pedal into the back of the booster. I have also read that the booster rod can be adjusted. One requiring an invasive grind and the other using hand tools. Some claim the rod adjustment at the pedal doesn’t affect MC preload.

Appreciate your guys help.

-Kevin

[weq92f]

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Looks like most or many boosters have adjustable pushrods (MC side). The one installed in my truck has a removable pushrod but it most definitely is not adjustable. My MC has a "slug" installed in the cavity of the piston to convert it from manual to power configuration and it too isn't adjustable ( see pics ).

The diagram above shows an adjustable booster pushrod. Since my booster doesn't have this feature, where do I adjust the length of this to remove MC preload being introduced by the booster rod?

pictures are as follows: booster pushrod and master slug

-klb

[72SB]

I believe MC are specific to power or manual, not aware of with or without filler slug unless the appropriate MC for power needs one (has recessed hole for it)

My manual to power conversions in my 67 and 72 I had to get "power" MC and booster combo. Each came with adjustable PR (threaded) to adjust at pedal end. I had to cut the threaded rod some on the Right Stuff kit but not the CPP kit. Cutting the rod was no big deal and I assume the kit had multiple applications, so they made the rod longer than my particular set up.

As mentioned, you need about 1/4" of free pedal travel before it engages the MC through booster so no pressure remains on the MC with foot off the brake pedal

[RichardJ]

Every one I have dealt with was adjustable. Must be aftermarket/Chinese.
You could grind it down on a grinding wheel, shorter and with the same contour. If you ever replace the MC, you might have to weld the pin back up to meet that situation.
What a pain.
A permanent flange type gasket spacer between the MC and booster would work. Aluminum sheet would probably be best. Soda/beer cans make good shim material and can be cut with scissors.

[cj847]

Originally Posted by weq92f

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Looks like most or many boosters have adjustable pushrods (MC side). The one installed in my truck has a removable pushrod but it most definitely is not adjustable. My MC has a "slug" installed in the cavity of the piston to convert it from manual to power configuration and it too isn't adjustable ( see pics ).

The diagram above shows an adjustable booster pushrod. Since my booster doesn't have this feature, where do I adjust the length of this to remove MC preload being introduced by the booster rod?

pictures are as follows: booster pushrod and master slug

-klb

Caution: May not be the best idea ever but worked for me.

I had your exact situation. I found you could order about 3 lengths of push rod, but none would have been a drop in fix. What I did (as it is difficult/impossible to grind the proper length and shape onto the rod with just a grinding wheel) was stack some washers on the mounting bolts on front of the booster. This held the master cylinder out just enough to eliminate the drag.

Note: I ordered and used the special measuring tool and also measured my washers with a micrometer to keep them the same distance side to side. There is a fine line between not enough space and too much space.

[weq92f]

Originally Posted by 72SB

I believe MC are specific to power or manual, not aware of with or without filler slug unless the appropriate MC for power needs one (has recessed hole for it)

<snip>

As mentioned, you need about 1/4" of free pedal travel before it engages the MC through booster so no pressure remains on the MC with foot off the brake pedal

.

I believe this kit is from CPP as their docs mention the slug as being their way of making it possible to install either power or manual MC. Yes...the free pedal travel. I found I have none.

Originally Posted by RichardJ

You could grind it down on a grinding wheel, shorter and with the same contour. If you ever replace the MC, you might have to weld the pin back up to meet that situation.
What a pain.
A permanent flange type gasket spacer between the MC and booster would work. Aluminum sheet would probably be best. Soda/beer cans make good shim material and can be cut with scissors.

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Grinding isn't going to be possible due to the shape of the tips. If necessary, I would consider the spacers/shims. I've seen some of these in pictures that looked good.

Originally Posted by cj847

Caution: May not be the best idea ever but worked for me.

I had your exact situation. I found you could order about 3 lengths of push rod, but none would have been a drop in fix. What I did ... was stack some washers on the mounting bolts on front of the booster. This held the master cylinder out just enough to eliminate the drag.

<snip>

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Agreed, this approach should be fine just as you have proven. However I think my problem is under the dash panel.
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Big shout out to Sheepdip and RichardJ for pointing out early in this thread that I'd need to first check the free play at the pedal. Earlier today, my friend and I found that brake pedal movement immediately moves the rod at the MC. I'm hopeful that adjusting the rod under the dash will bring that booster rod back so that it isn't preloading the MC.

Thanks gentlemen.

-Kevin

[pjmoreland]

You may also need to adjust the brake light switch if it is preventing the brake pedal from retracting.