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

.

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.

[weq92f]

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Got the cotter pin out of the clevis roll pin after 90 minutes cussing at it. After observing things for a while I've come to question the whole free play at the pedal notion. Everything is rigid once connected. The rod will not pull out of the booster from inside and any forward movement of the rod actuates the MC. It is a rigid coupling from MC to booster to brake pedal. There is no possibility for free play. Maybe the fraction of an inch that exists at the clevis roll pin to brake arm contact point, but I can't see where any free play would be found here.

One positive note is that removing the clevis from the brake pedal arm caused the booster rod to recess into booster 1/8" roughly. Whether that's enough to stop my brakes from locking up is questionable.

If I bring the now unconnected rod down 1" from the original drum brake hole it falls almost right in the center of the pedal arm right now. According to the document above, power brakes need more stroke and to get that, make a new hole 1" below the original hole (when going from drup to disc) and attach there.

So...as it appears removal of the brake pedal assembly isn't something I want to do right now (can't drill it installed), ima gonna remove the booster so I can spin the clevis in 1/8" and attach it back to the same spot and see what I get. Looking for no drag and full brake power somewhere before pedal hits floor. If there's an easier way to get the rod away from the brake pedal shaft...let me know please.

Have any of you drilled the new hole in brake pedal arm when going from drums to disc brakes on these trucks? Why is it that my system has zero free play...where would that free play exist in the linkage?

-Kevin

[weq92f]

Originally Posted by pjmoreland

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

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Got eyes on the switch today. The pedal is on the rubber stop and the tab on the pedal is resting on top of the switch holding the switch down.

-klb

[72SB]

There is a different position for rod on brake pedal for power brakes. If yours is not already drilled, use a right-angle drill and drill a new hole.

If you do have a threaded clovis, pull booster back so you can turn it to shorten rod length. Just make sure there is then not to long of rod (threaded end) that then hits the brake pedal arm with clovis on. If so you need to cut the rod down, threaded end.

[weq92f]

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First iteration: spun the clevis toward firewall about 1/2 inch. This pulled the pedal about an 1/8th off the rubber stop and completely eliminated brake drag. However now the pedal hits floor before max brake application.

The plan now is to adjust rod length back to where it was (3 full turns back toward dash) plus another turn and a half. Then either cut down or eliminate the bump stop in order to remove the drag AND still allow max brake before pedal hits floor.

-Kevin

[pjmoreland]

You shouldn't need anywhere near that much pedal travel to fully engage the brakes. Maybe your rear drum brakes need to be adjusted. Either that, or now you have excessive clearance between the rod and the master cylinder.

The brake light switch might not be able to be adjusted far enough to allow your pedal to retract farther than the rubber bump stop

[weq92f]

Originally Posted by pjmoreland

You shouldn't need anywhere near that much pedal travel to fully engage the brakes. Maybe your rear drum brakes need to be adjusted. Either that, or now you have excessive clearance between the rod and the master cylinder.

The brake light switch might not be able to be adjusted far enough to allow your pedal to retract farther than the rubber bump stop

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All of this is possible for sure! This d@mn thing has been fighting me every step of the way on this adventure.

The brake switch is indeed a problem right now as my friend and helper pulled the brake pedal right through it, crushing part of the switch and bending its metal mounting tab. I was going to wait to repair that until I got the brakes where I want them, but you bring an interesting point I hadn't considered. Will it adjust as far as I'm moving the pedal arm? Oh boy...another potential problem area!

Thanks,

-Kevin

[weq92f]

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Second iteration:

Removed bump stop, turned clevis toward dash to where the brake pedal now rests inside the space the bump stop occupied previously so the rod has been lengthened an additional 1/2 over its previous length. The brake switch appears to have plenty of adjustment.

Adjusted rear shoes. One side was way off the other was just slightly misadjusted. Bled all 4 corners from furthest to closest to MC.

The pedal feels good and the brakes respond but I can't get full braking power before the pedal stops its motion toward the floor. It's better than it was this morning but still not safe for the road.

Looks like MC and PV replacement next.

-Kevin

[72SB]

power brakes use a 1 1/8" bore MC
Manual brakes use a 1" MC

The bigger bore allows for a shorter pedal stroke.

[cj847]

One way to think of the whole system:

The center of the brake booster is the center of the brake universe (everything has to be adjusted to the brake booster's neutral position).

Adjust clearance of master cylinder to the booster; then adjust your pedal throw inside the cab; then adjust your brake light switch.

Sounds easier than it is, as you are finding out.

[weq92f]

Originally Posted by cj847

…<snip>…

Sounds easier than it is, as you are finding out.

.

Indeed.

-weq

[weq92f]

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The new master cylinder came with a deep recess in the piston, just like the old one. But the slug from the old MC didn't fit into the hole. Ended up purchasing a new booster rod of the "long" variety, which is what's used with the deep recess/pocket MC. That was a 3 hour round trip. Got it back to the shop and found it would not fit into my booster! It was very close as the caliper showed and so I chucked it into a drill and rubbed the booster end on 20 grit sand like I was honing a cylinder then finished it with 800 grit. Took a bit but it fit.

Put the rear up on stands tonight and the plan is to bleed the brakes in the morning using pedal power then head out to the big show at TMS. I'm hoping for full braking power before the floor once again!!! If not...truck stays home.

Picture shows the original short booster rod and the one I installed after modification.

-Kevin

[weq92f]

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MC must have gone bad as we got brakes now...full on front and rear brakes!

Back in business!

Note: The new MC did not come with a bench bleed kit, instead it came with instructions as follows:

Get the MC in a vice and level it.
Fill reservoir with fluid.
Wait until fluid is steadily flowing/dripping from output ports.
Place output port plugs into output ports
Depress piston slowly into MC no more than 1" and...
Slowly release piston
Wait 15-20 seconds
Repeat piston in 1" slow then out slow
Continue until no bubbles seen in reservoir.

Well, this did not work as advertised. I ended up fabbing up my own bleeder tubes from output ports to reservoir and bled it the old fashioned way!

-Kevin