Soft brake pedal when running

[Oldironracing]

Everything is new. Willwood master 11/8 bore. Factory front disc and factory rear drum all is new. Dual 7” booster new. Have bled everything several times.
The pedal is hard and feels good when not running. Start the truck and the pedal is soft and goes almost to the floor but I do have brakes. Have not drove truck to test. Something just don’t feel right.

[LockDoc]

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Try this on the rear drums and see if it helps.

It all begins with adjustment of the brake shoes.

On the drum brakes, one thing I found on mine is when you put everything back together and adjust the brake shoes you can’t just adjust them out until they start rubbing and quit. You have to adjust them out as far as they will go and you cannot turn the wheel or drum at all. This centers the whole assembly in the drum. (I even tap around the drum as I tighten them to help them center themselves) Then you back them off until the wheel or drum just turns, and stop. If you only adjust them out until they start rubbing and quit, the first time you press on the brake pedal the brake shoe assemblies will center themselves and you will have too much clearance between the shoes and the brake drum. If there is too much travel in the shoes the pedal will go down too far before the shoes contact the drums. I also put three lug nuts on backwards to hold the drum tight against the axle on the rear before I adjust them.

I know it is a pain in the butt to try and hold the self adjusters off of the adjuster wheel while you back them off, but this is the only way I could get my drum brake shoes adjusted correctly.

LockDoc
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[Oldironracing]

I will try that tonight after work and get home. Thanks.

[Longhorn 69]

Good info Leon, I will have to do that process to mine. I assume I should do that to the front drums also.

[Caddylackn]

There is supposed to be a pressure residual valve PRV on the master cylinder from the factree for the drum side. The PRV holds 5- 6 psi in the line to take up all the slack in the drum brake linkage and to help account for the additional volume of fluid needed for wheel cylinder pistons compared to caliper pistons for disc brakes. An all drum master cylinder, there is a PRV on both circuits. A disc/drum set up should only have a PRV on the drum circuit. No PRV is required for disc since they are so efficient on converting hydraulic pressure to brake force. The PRV is just a shaped rubber plug with a spring under the seat for the brake line. You can check it by putting a small screw driver into the opening and see if it moves under light force.

With a PRV on the rear circuit installed, the amount of pedal travel needed to engage the rear brake drums is less.

You can also tighten your rear brakes by slowing going in reverse and applying the brake several times. The factree brake hardware is set up to bump the adjustment wheel a notch every time you hit the brakes in reverse if there is adjustment required.

[LockDoc]

Originally Posted by Longhorn 69

Good info Leon, I will have to do that process to mine. I assume I should do that to the front drums also.

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Yes, same procedure.

LockDoc

[RichardJ]

>>You have to adjust them out as far as they will go and you cannot turn the wheel or drum at all. This centers the whole assembly in the drum.<<

That's all well and fine, but remember that any manipulation of the star adjusting wheel can move the shoes around.
Simply apply the brakes to center the shoes. Adjust the star wheel and recheck for drag by applying the brakes again. Repeat as needed. Any slight, intermittent drag will wear down after a few miles.

You can make of this what you will.
You won't find Residual Check Valves in very many MC's being sold today.

The purpose of the Check Valves is to make sure air doesn't enter the wheel cylinder when the drum springs push the cylinder pistons back into the wheel cylinder.

New and rebuilt drum wheel cylinders being sold today have improved cup expanders than the ones used 50 years ago.

[LockDoc]

Originally Posted by RichardJ

>>You have to adjust them out as far as they will go and you cannot turn the wheel or drum at all. This centers the whole assembly in the drum.<<

That's all well and fine, but remember that any manipulation of the star adjusting wheel can move the shoes around.
Simply apply the brakes to center the shoes. Adjust the star wheel and recheck for drag by applying the brakes again. Repeat as needed. Any slight, intermittent drag will wear down after a few miles.

If you adjust them out as far as they will go and you cannot turn the drum at all then back them off until the drum just turns, there is no where for the shoes to move to. The shoes are still close enough to the drum that they can't possibly move around.

LockDoc

[Oldironracing]

The rear brakes are adjust did what was suggested. Still soft pedal when running.
Feels like to much help from vacuum booster.

[Oldironracing]

Where is all the Tec guys that have been there done that with the helpful tips.

[Richard]

IDK, if I were in your situation with the pedal going to the floor I would concentrate on air in the system. To much assist or brake pedal pressure would not cause that.

[Lethal Tendencies]

I would try driving the truck in the driveway and see if it gets better. How are you bleeding the brakes. Power Bleeder, gravity, or someone pushing on the pedal ?

[Oldironracing]

I have bleed the brakes every way you can even bought a reverse bleeding tool at 400.00. I have took the master cylinder of bench bleed it of and on for two days at night after work. Put a 10lb residual valve in the rear brake line and reinstalled master. Bleed the hole system agin and bleed it six times. Still have the same problem. There are no leaks.

[Blazerowner]

Did you ever solve this problem? I’m having the same problem. I haven’t tried to adjust the rear shoes as I thought they self adjusted. I jacked the rear up and spin the tires and get brakes when I push the pedal. No issue when it’s not running. As soon as I start the engine and there is vacuum the pedal wants to go to the floor.

[rockyrivermark]

I had same problem and drove me nuts but I did solve it.
I’m not sure if you had a disc brake conversion but for me I had the calipers backwards from left side to right. This out the bleeders in the down positions so they wouldn’t bleed. Make sure the caliper bleeders are in the top position

[geezer#99]

Originally Posted by Blazerowner

I haven’t tried to adjust the rear shoes as I thought they self adjusted. .

They do self adjust but only if you back up and hit the brakes.

[Sheepdip]

Originally Posted by rockyrivermark

I had same problem and drove me nuts but I did solve it.
I’m not sure if you had a disc brake conversion but for me I had the calipers backwards from left side to right. This out the bleeders in the down positions so they wouldn’t bleed. Make sure the caliper bleeders are in the top position

^^^^^^^^^^This is a very common problem upon installation^^^^^^^^

[Blazerowner]

I wasn’t aware you had to back up to adjust. I’ll have to try adjusting them out to see if that helps. My calipers are correct with the bleeders on top. It is stock with disc front drum rear. The whole system is new from master to drums and calipers and all new lines. It has braided stainless hoses.

[Oldironside]

Sounds very familiar. In my case I had to adjust the master pushrod length. It took me quite a while to find out that this was the problem.
Here is an article
https://autoedu.info/en/auto-mechani...ushrod-length/

[Blazerowner]

This caused the same issue? I haven’t checked the master but I bought the whole kit from the below thread. Maybe I was wrong to assume it was bolt in? The instructions do not mention adjusting the push rod.

http://67-72chevytrucks.com/vboard/s...d.php?t=581862

[geezer#99]

That pushrod might be fine but have you changed your pedal ratio? Sometimes that will help.



https://techtalk.mpbrakes.com/how-to...ng-pedal-ratio

[Oldironside]

Yes, same issue and my parts (in fact the whole brake system) were brand new. Checked every brake line, made sure there was absolutely no air in the system, and so on.
I can't tell if your problem has the same cause but wanted to give you a hint if everything else fails.
Best of luck to you.

PS: Forgot to mention that I did not use any fancy toosl. Just by try and error. Moving the screw in tiny sequences. It actually was not that big of a deal but it was a timeintesive task that tested my patience.

[Blazerowner]

I’ll have to look at all the options. I appreciate you guys throwing ideas out. I’ll update if adjusting the rear drums works. If not I’ll move to the master and rod adjustment. I didn’t change the pedal ratio, but I did buy a Lokar brake pedal. It appears everything is in the same position as stock.

[Steeveedee]

How far does the pedal actually move when the truck is not running? If your hard pedal is at the correct height, it's likely that the pushrod is properly adjusted.

[Blazerowner]

It has some play which makes me think the slight gap described in the link above is there. When not running I have a firm pedal. Moves maybe an inch and I have brakes. I’ve lifted the rear and spun the tire while I push the brake and it grabs. I can also see the front calipers flexing when pushing on the brakes while I was bleeding them. It’s only when I start it and have vacuum I have mushy pedal. It feels pretty hard but gradually pushes almost to the floor. Everything I have read says this is indicative of air. I just went out and adjusted the drums to where they are touching and still the same. So my next step is I ordered the measuring tool and will check the rod length when it shows up. If that doesn’t work I’ll try vacuum bleeding when it arrives as well. I have to say I’ve never had this issue. My Motive pressure bleeder had always worked great. I’m sure I’ll figure it out just have to have patience.