Help... Soft Brakes.

[vossy]

I need help... I'm at my wits end with these front disc brakes.

After my upgrade to disc brakes... I'm struggling to get the hydraulic system for new front disc brakes to bleed out properly... I'm confident there is no air in the brake line. I've bled them so much, I think I have literally put 6 bottles of Dot 3 through the system... But, no matter what I do, the brake pedal is soft for the first 75%, and firms up when it gets an inch or two off the floor. I have no apparent fluidleaks... And Iim
Not sure what else causes a mushy pedal.. So I'm getting suspicious of the suspicious of the junkyard proportioning valve..? Can anyone tell me if looking at proportioning valve makes sense, or am I missing something.

Additional detail: It seems that when I open the rear bleeder screws, the firmness I mentioned at the bottom of the brake pedal press goes away, meaning it's totally spongy. The same change is not felt when one of the front bleeder screws is loosened. Help!

[fordman]

My very first thoughts are. Bad or wrong master calipers on the wrong side. You must use a disc brake master. Or remove the deals inside the drum master to get right amount of pressure to the brakes. And i cant remember if its front or back you remove it from. But i think its the front. The caliper bleed valve must be in the top postion to bleed the air.

The proportioning valve makes proper fluid pressure to each brake in a equalizing way. You could try bleeding it to center that valve.

Air and improper pressure are the two fail points to look at.

Are the rear shies adjusted out properly? That happened to me. As well as improper balance of pressure.

[fordman]

http://www.fordification.com/tsb/viewto ... p=696#p696

[BobbyFord]

Try a brake pressure bleeder. Sounds like you still have some trapped air.

[ultraranger]

Did you reuse the old MC from your drum brake system or did you install a new disc/drum MC. If the MC was replaced, did you bench bleed it before putting it on?

Is the brake system manual brakes or does it have a brake booster?

Is the wiring connector to the switch, sticking out of the brake valve, plugged in and if so, is the brake warning light on the dash lit up?

The port on the MC closest to the radiator is the feed to the rear brakes. The MC port closest to the firewall is the feed to the front brakes.

[vossy]

Answering some of the questions...

I used 2 different pressure bleeders. A mighty vac, as well as a compressor powered system from harbor freight. I'm like 99% sure there is no air in these lines.

I'm using the master cylinder and booster from the donor '74.

I have the instrument console on this truck torn apart, so I'm not 100% sure if the brake warning light would be on or not, but when I put the two ends of a continuity meter/voltage across the posts on the valve warning switch, it does show as a completed circuit. So I think the warning light would be on...?

I did bench bleed the MC by looping two lines from either output back into their respective MC reservoirs and pumped until there was no air coming out and like 40 more times for good measure.


I have the smaller reservoir output (nearest radiator) going to the rear brakes and the larger reservoir output (nearest the firewall) going to the front.


Some additional details that might help...

When I depress the brake quickly as if in a panic stop situation the break feels great... And retains that pressure for a few minutes as long as you press the brake down quickly. But press it slowly and I will go almost all the way to the floor, with just a little pressure at the bottom.

[fordman]

Sounds like the brakes are unequal. Out of balance. Or the valve is in need of a rebuild. They only use a few o rings.

[vossy]

What does unequal mean? I mean I know what the word unequal means... But how can the brakes be unequal? In what way?

And how do you fix it?

Would a spongy brake pedal be a symptom of a bad valve?

[fordman]

Ultraranger posted a good post in this thread. http://www.fordification.com/forum/view ... ve#p682964

But after rereading the tech link above i bet the master is worn out or pitted and leaking past the seals internally. But you still have to center the valve when done.

[ultraranger]

Personally, I only use NEW MCs. I don't buy rebuilt MCs and I definitely don't run wrecking yard MCs. This is a recipe for bad things to happen.

Most likely, and especially if you jabbed the brakes on during the brake bleeding process, the pressure differential valve spool inside the disc/drum brake valve has probably shifted. It will need to be re-centered.

If you read continuity off the switch pins, this is a good indication the pressure differential valve spool is shifted.

[vossy]

Thanks for all the replies.. I read the ultra's post on centering the valve, and will give that a shot as soon as I got a few spare moments.. I have a few more questions about the proportioning valve (if anyone feels like indulging), when it's in the 'tripped' state.. What is it actually doing, other than turning on a light? Is it affecting the way the hydraulic system works, or diverting the fluid in some manner, perhaps away from the side of the brake system that it thinks is damaged? Apologies if that's a stupid question, but I'm not sure I've seen that explained, and it's not readily apparent, to me anyway, when looking at the schematics. I mean I realize a shift in pressure moves a plunger, which turns on a light, but is there something else going on inside that proportioning valve, when it trips either way?

Also, a general question about these brake systems, that I've been wondering.. Are the two hydraulic systems (front and rear) entirely seperate? In other words could you open a bleeder screw on the front brakes, and pump them a few times, sucking some air into the front lines, and then close that screw. Would/Could that air introduced in the front system ever be able make it's way into the rear hydraulic system? In my mind, the only point of cross contamination would be in that black box (to me), of a proportioning valve...

Thanks again for all the replies, I really appreciate this community and you all have been a huge help in getting this old farm truck back on the road...

[fordman]

On a good working valve once its tripped. The fluid from front and back cant cross between the two.

When it is tripped it seals off the tripped or leaking side. So brakes will still work on the good side. Whether that be front or back.

[ultraranger]

I just noticed what you said earlier about reading continuity ACROSS THE PINS of the switch....

If you took your meter leads and placed one each on the two pins sticking up in the switch on the valve, you would show a completed circuit but, this would be a false reading.

There are two pins where the connector plugs in but, if you look down into the switch you'll notice there's a small bar that links the two pins together. This means the two pins are (electrically) at the same potential.



If the pressure differential valve spool is shifted, it will complete a path to ground through the BODY of the valve assembly. Place one meter lead on one of the pins on the switch (doesn't matter which one --they're tied together), and place the other meter lead to the body of the valve.

If the spool is shifted, you will read some value. If your meter has an audible (Beep) feature and the meter beeps, it means the circuit is completed and that the spool is shifted. (contacts closed).



If the spool is centered (as it's supposed to be), you will read infinite resistance (OL --Open Line) or, if your meter has the audible feature, there'll be no 'Beep.' (contacts open).



The second wire off the brake valve switch is for the purposes of 'prove out.' ('68-up models. '67 only had a one wire circuit to the pressure warning switch). The 2nd wire is tied to the ignition circuit. When you go to start the vehicle and turn the key to the 'Start' position, the red brake warning light on the dash will momentarily light up. After the key moves to the 'Run' position, the brake warning light will go out. This is just to let you know that the bulb/circuit is working. If the warning light stays on, there's a problem in the brake system that needs to be found and corrected.

--If you were in the wrecking yard looking for a brake valve from a donor vehicle, it's not very likely you're going to find a vehicle with a good battery AND the ignition key in it, to see if the brake valve is shifted, by looking for a lit brake warning light. The meter check I described above can be done in the field to determine if the valve is shifted, without the circuit actually being powered up.

[vossy]

Wow, you're awesome, great post. Thank you! I was getting a false positive the way I was doing it.. I checked, and it's not completing the circut against the body of the valve.

I backed the switch out, and made sure it was working, by depressing the plunger on the bottom of the switch and checking the circuit from the pin on top of the switch to the plunger on the bottom, when the plunger is not compressed, circuit is not complete, when the plunger is depressed the pins on top of the switch make a complete circuit with the plunger... So presumably the switch is working.

I didn't have time this morning to really make sure I had a good ground on the valve body, with one end through, I tested it in a few spots. When I get some time later today, I will get out a little wire brish and make sure I have nice shiny section of brass exposed on the valve to make sure...

I did try to look down into the hole, after removing the switch, but it's pretty hard to see in there, it's a weird angle and a tiny hole, I can see where the plunger from the switch slides down, but not knowing what it looks like in the shifted or nonshifted position, so it didn't help much looking down the hole.. (I have it mounted on top of my steering box, like it was in the '74)


Incidentally, the valve you have pictured here looks like the one I took out of my '71, which I thought I read somewhere was just a distribution block, it doesn't look at all like the valve from the donor '74. Can I use the valve, from the '71 that looks like the valve you have pictured here? I think it's lying around my garage still somwhere?

[fordman]

No. The proportioning valve is for use with disc brakes only. The distrubution block is for drum brakes only.