Brakes Issue

Just got a fresh built 28 Ford truck. It has new discs all the way around with new MC & booster.With the motor off I have great pedal after bleeding. When I start the motor the pedal goes way down and sticks there. Almost like the booster is not working. The hose to the booster is pluged into the intake but the modulator valve to the tranny is also pluged in the same place. Would this cause the booster not to work. Or is my new booster bad. thanks in advance for any help.

 

Anybody?

 

Still trying to figure it out. Unhooked the mod valve.Still the same.If booster was bad I would have hard pedal, Correct?

 

the booster should have a dedicated ( Lone ) source.

 

Thats what I thought. But if you unhook the mod valve and plug off extra hole still have the same issue with brakes. Starting to think MC

 

The "return spring" for a power-assist booster is in the booster. If the pedal drops on it's own after starting the engine, then the valving in the booster is bad... it's allowing engine vacuum to over-ride that return spring. Some cars still had an external return-spring in addition to the one in the booster.

If it just has a low pedal and you are absolutley sure there is no air in the system, I would look at the brake adjustment.

 

if it has a check valve on the the booster make sure it is working

 

The pedal doesnt drop by itself. When you start the truck and push it down it stays. The thing that plugs into the booster for vacuum doesnt have a valve in it. Again it works fine until you start the motor. Theres pics of the set up in my albums under 28 Ford

 

Try adjusting the m/c pin shaft, The pin shaft should just be touching the booster piston . as little as 1/32 space can give you a soft pedal under boost . good luck

 

Thanks, I"ve give up for the night, but will try it tomorrow.

 

Ok Adjusted the rod and the pin shaft. Still the same thing. Two things Ive noticed. If I pump the brakes while motor is not running,I have great brakes.I hold them and start the motor they dont go down but they dont return to the top. They return to the top with engine off or if I disconect the brake booster hose. I also replaced the check valve with a new one. Any ideas?

 

There is also a swish when you disconect the booster. Meaning it holds air.

 

darrellwv, what size is the booster?
Is it a standard booster off of a car or a custom one?
Do the brakes stay engaged when the pedal stays down?

 

7" New booster bought off ebay.There not great breaks, but yes the stay engaged when the pedal stays down.

 

Are you sure you have the correct pedal ratio? I may be way off but it's free to check.
I can't recall what a booster does if you have your foot on it when you start the car but being that the "boost" capability is activated when you start the car it's gonna go up or down. My logic says it will go down since the same amount of leg pressure should give you more brakes when assisted. But I'm late for bed and my logic is weary.

 

The internal piston in the booster is sticking. Check to see if there is any brake fluid or gas in the booster.

 

There is none that I can see. Would it be coming out somewhere? Is there a way to unstick it.

 

Check the pedal ratio, as already stated, to be sure it's around 3:1.

I've never seen or worked with a booster that didn't have the pedal push rod factory installed and non-adjustable in the valve body. The push rod might be adjustable to the pedal, but not to the booster.

The booster push rod can be adjustable to the master cylinder, and should be .035"-.040", checked with full vacuum and pedal FULLY released.

The pedal should have it's own return spring, and not relay on the booster. The internal spring is designed to return the diaphragm(s), not pedals and linkage.

I viewed your project, and it is VERY nice, but too early to finalize the pedal position. This has to be done when the body, floor, etc. are in place. You'll want to be sure the pedal does NOT bottom out before the master cylinder, at that time. But for now rig a return spring on the pedal and see if the booster works properly.

 

Pardon me for being uninformed about pedal ratio.I dont understand what you are talking about. I googled it and still in the dark.Will go back and read some more to try and understand it.I unhooked the brake lines from the MC and pluged them. Pumped the brakes up then started.(Very good pedal) Pedal drops slighty as it should on start up.However still have the same issue as before. When started and pressing pedal it does not return. I have adjusted the rod coming from the pedal assembly going to the booster by only allowing 1/4" play in the pedal.I have plenty of adjustment to go either way with the floor in it. I also adjusted the rod coming from the booster to the MC. I dont remeber if it was at full vacuum or not. I didnt do it with the engine started if thats what you mean by full vacuum? The spring for the rod would be hard because of the set up, but I think it can be done.
Thanks for the compliment and the advice.

 

I don't know how you Googled, but if you type "brake pedal ratio", you'll get many links, including the one below.
Yes, full vacuum means about 20" Hg with the engine running, unless you have a hand held vacuum pump.
What "spring for the rod"? The return spring should be connected to the pedal, either above or below the floor.
Your pictures don't show much of the pedal/booster assembly-maybe a better shot or two would be helpfull.

http://www.mpbrakes.com/uploads/documents/pedalratiopdf.pdf

 

After I posted I found the link you provided. If I understand this right,(4 to 1)Example: the pedal goes down 4"s the rod goes in to booster 1". Is this correct? If so how do you change it if its not correct. If I adjust the rod in or out going to the booster all it seems to do is change pedal postion

 

I added more detailed pictures of the brake system in my album (28 Ford)