Brake pedal low what now?

[FuzzFace2]

Little back ground: 81 F100 300 six, T18 trans.

Truck started with manual disc front, drums rear. (note started as it may play a roll?)

Everything from the master to all the wheels are new and for the manual brake truck.

New: calipers & pads, all 3 rubber hoses, new drums, wheel cly, spring kits

Now after checking parts (caliper piston size & wheel cly size) I used the power booster, master and pedal assy. from the parts truck turning it into a power brake truck.

I just replace the booster as it was leaking and the master would go to the floor so it was replaced at the same time. Master was benched bled and was happy it showed no air after doing this.

I also measured the rod out of the booster both old and new and the new one looked to be a little longer.

When moving the truck, both before and after the booster / master change, the pedal is vary low like almost to the floor. It felt better after the booster / master change but still not right.

Had the wife help bleed the system, not running pump 3 time and hold. Did not get any air but it also did not squirt out vary hard? I also checked the drum adjustment and they are good.

After this the pedal felt worst if that can happen.

The pedal dose not pump up like if the drums were out of adjustment or air in the system but the pedal does feel mushy and can bottom out the master.

In the morning going to bleed the system again with motor running to see what that gets me.

At this point I am at a loss as what could be the problem.

Does someone see a problem I don't? Did I over look something?

Thanks for any help you can give.

Dave ----

[Gary Lewis]

Dave - The rod between the booster and the master is a major player in this, and its adjustment is critical. WelderScott's '78 had the pedal almost to the floor and an adjustment of the rod fixed it. But, I will say that you do not want to make the rod too long, as we did that initially and the brakes started dragging.

The adjustment is described here: Documentation/Driveline/Brakes and then the Master Cyl's & Boosters tab, then the Push Rod Adjustment tab.

Also, on the same page, go to the Overall tab and then the General Brake Service tab. There are several things to test there, but I think this one may be key:

Power Brake Function Test

1. With the engine stopped,eliminate all vacuum from

the system by pumping the brake pedal several

times. Then push the pedal down as far as it will go,

and note the effort required to holdit in this position.

If the pedal gradually moves downward under

pressure, the hydraulic system is leaking and

should be subjected to a hydraulic leak test.

2. With the brake pedal pushed all the way down, start

the engine. If the vacuum system is operating

properly, the pedal will move downward. If pedal

position does not change,the vacuum system is not

operating properly and should be subjected to a

vacuum system test.

[FuzzFace2]

Dave - The rod between the booster and the master is a major player in this, and its adjustment is critical. WelderScott's '78 had the pedal almost to the floor and an adjustment of the rod fixed it. But, I will say that you do not want to make the rod too long, as we did that initially and the brakes started dragging.

The adjustment is described here: Documentation/Driveline/Brakes and then the Master Cyl's & Boosters tab, then the Push Rod Adjustment tab.

Also, on the same page, go to the Overall tab and then the General Brake Service tab. There are several things to test there, but I think this one may be key:

Power Brake Function Test

1. With the engine stopped,eliminate all vacuum from

the system by pumping the brake pedal several

times. Then push the pedal down as far as it will go,

and note the effort required to holdit in this position.

If the pedal gradually moves downward under

pressure, the hydraulic system is leaking and

should be subjected to a hydraulic leak test.

2. With the brake pedal pushed all the way down, start

the engine. If the vacuum system is operating

properly, the pedal will move downward. If pedal

position does not change,the vacuum system is not

operating properly and should be subjected to a

vacuum system test.

Thanks Gary,

Thing is I used the parts truck master / booster and never took the 2 apart and it had the low pedal but I know the master was junk as it did sink to the floor with foot on it and if you pumped it up the booster would start hissing and motor start running ruff, vacuum leak so it was bad also.

I think between the master and booster the PO lost brakes and why it rolled.

This new setup does not sink and no vacuum leak if pumped.

I did measure the rods between the 2 units and the new one had a longer rod.

Also that rod adjustment looked out pretty far and don't know how much more I can go.

In a bit we are going to do another bleed with motor running and if that does not pan out I will make the adjustment, cant hurt at this point.

Dave ----

[FuzzFace2]

Dave - The rod between the booster and the master is a major player in this, and its adjustment is critical. WelderScott's '78 had the pedal almost to the floor and an adjustment of the rod fixed it. But, I will say that you do not want to make the rod too long, as we did that initially and the brakes started dragging.

The adjustment is described here: Documentation/Driveline/Brakes and then the Master Cyl's & Boosters tab, then the Push Rod Adjustment tab.

Also, on the same page, go to the Overall tab and then the General Brake Service tab. There are several things to test there, but I think this one may be key:

Power Brake Function Test

1. With the engine stopped,eliminate all vacuum from

the system by pumping the brake pedal several

times. Then push the pedal down as far as it will go,

and note the effort required to holdit in this position.

If the pedal gradually moves downward under

pressure, the hydraulic system is leaking and

should be subjected to a hydraulic leak test.

2. With the brake pedal pushed all the way down, start

the engine. If the vacuum system is operating

properly, the pedal will move downward. If pedal

position does not change,the vacuum system is not

operating properly and should be subjected to a

vacuum system test.

Thanks Gary,

Thing is I used the parts truck master / booster and never took the 2 apart and it had the low pedal but I know the master was junk as it did sink to the floor with foot on it and if you pumped it up the booster would start hissing and motor start running ruff, vacuum leak so it was bad also.

I think between the master and booster the PO lost brakes and why it rolled.

This new setup does not sink and no vacuum leak if pumped.

I did measure the rods between the 2 units and the new one had a longer rod.

Also that rod adjustment looked out pretty far and don't know how much more I can go.

In a bit we are going to do another bleed with motor running and if that does not pan out I will make the adjustment, cant hurt at this point.

Dave ----

BTW do you remember when you made that adjustment the before & after measurements?

My book shows .9998" from mounting base to tip of rod.

The old unit was about 1" the new looked to 1-1/8" or so.

Dave ----

[Gary Lewis]

BTW do you remember when you made that adjustment the before & after measurements?

My book shows .9998" from mounting base to tip of rod.

The old unit was about 1" the new looked to 1-1/8" or so.

Dave ----

Dave - I don't understand your question for sure. But I think you are asking what the before measurement was on Scott's rod. If so, the answer is that we didn't measure it. But we didn't have to turn the rod out very far for it to make a big difference. However, we found the adjustment critical as just a teeny bit too much had the brakes dragging, then then heated up and dragged more, so they got hotter, and pretty soon they stopped the truck.

What book has .9998"? The .995" comes from the 1985 factory shop manual, but I compared it to the 1981 FSM and it was the same. So I believe it, and I wouldn't go the extra .005" your book suggests. At the time we did the adjustment we used my dial calipers to attempt a measurement, and that wasn't easily done and we did go a bit too far.

[FuzzFace2]

Up date:

Bled the brakes again with motor running and think I got some air out but my helper cant tell if the pedal is better or not. Does not under stand if the pedal will pump up or has come up from where it started

It did not I cut the helper loose.

I then checked the rod out of the booster to master to see if that is the issue.

It was sticking out 1-1/8" and the book calls for .9998"?

So going to adjust it out more to see if that will bring the pedal up some.

Good thing I checked as the jam nut was loose and you could spin the adjustable tip.

Run it out to 1-1/4" but was too long, held the master out some so turned it in a little but still out more than 1-1/8". This 1/16" did nothing for the low pedal

Out testing in the drive and I can not get any of the tires to lock up, pedal is low and mushy?

Fronts tend to do more marking than rear.

I even adjusted the drum brakes up more to see if this would help.

I tried to pull and push the pin on the porting valve thinking maybe this was the reason for the low / mushy pedal?

When you push the pedal in it feels like the first half of the master is not doing a thing till it gets to the bottom half. Almost like when a line brakes and only half the system is working.

When bleeding I am using a clear hose on the bleeder to a plastic jar, took out about 12 oz between yesterday and today.

I think this new, not a rebuilt, master is bad and will look into getting it replaced.

I say this besides how the pedal feels but when bench bleeding I only had a good discharge from the small side not the large.

So I am at a total loss what could be the trouble with this system?

Dave ----

[Gary Lewis]

Up date:

Bled the brakes again with motor running and think I got some air out but my helper cant tell if the pedal is better or not. Does not under stand if the pedal will pump up or has come up from where it started

It did not I cut the helper loose.

I then checked the rod out of the booster to master to see if that is the issue.

It was sticking out 1-1/8" and the book calls for .9998"?

So going to adjust it out more to see if that will bring the pedal up some.

Good thing I checked as the jam nut was loose and you could spin the adjustable tip.

Run it out to 1-1/4" but was too long, held the master out some so turned it in a little but still out more than 1-1/8". This 1/16" did nothing for the low pedal

Out testing in the drive and I can not get any of the tires to lock up, pedal is low and mushy?

Fronts tend to do more marking than rear.

I even adjusted the drum brakes up more to see if this would help.

I tried to pull and push the pin on the porting valve thinking maybe this was the reason for the low / mushy pedal?

When you push the pedal in it feels like the first half of the master is not doing a thing till it gets to the bottom half. Almost like when a line brakes and only half the system is working.

When bleeding I am using a clear hose on the bleeder to a plastic jar, took out about 12 oz between yesterday and today.

I think this new, not a rebuilt, master is bad and will look into getting it replaced.

I say this besides how the pedal feels but when bench bleeding I only had a good discharge from the small side not the large.

So I am at a total loss what could be the trouble with this system?

Dave ----

You are sure the rears are adjusted up fully? If so, and if it still feels like the front is doing most of the stopping, when either the master is bad or the brake pressure differential valve may be stuck off center.

[FuzzFace2]

You are sure the rears are adjusted up fully? If so, and if it still feels like the front is doing most of the stopping, when either the master is bad or the brake pressure differential valve may be stuck off center.

On the valve is there a way to check this off center thing?

I have seen post the plunger needs to be pulled out for bleeding but it does not want to move.

I am going to build a cheap pressure bleed system to see if that helps.

Dave ----

 

[Gary Lewis]

You are sure the rears are adjusted up fully? If so, and if it still feels like the front is doing most of the stopping, when either the master is bad or the brake pressure differential valve may be stuck off center.

On the valve is there a way to check this off center thing?

I have seen post the plunger needs to be pulled out for bleeding but it does not want to move.

I am going to build a cheap pressure bleed system to see if that helps.

Dave ----

If the plunger is to one end the brake warning light on the dash is supposed to be on. Does it come on when you first turn the key on? It is supposed to in order to prove the bulb is good.

[FuzzFace2]

If the plunger is to one end the brake warning light on the dash is supposed to be on. Does it come on when you first turn the key on? It is supposed to in order to prove the bulb is good.

It does not but I am also running LED's and that one could be in 180* out?

The light on if the plunger was off center did cross my mind and even thought of shorting the wire out to see if it worked but pressed for time I skipped that. Something else to add to the punch list LOL

The only light that comes on is the seat belt light when key first turned. The turn & hi beam lights do work. How could I have missed the brake light?

I have to return the booster and the battery for cores and will check if they have a PSI bleeder I can rent if not I will look into making one. I started to years ago but think I can come up with something a little easier to build & use.

Thanks

Dave ----