Brake question

[steveg0690]

My 78 F250 is the longest legged vehicle that I've ever driven. For some reason It does not like to stop. It's like having manual brakes all over again.

So far I have replaced All the drums, rotors, pads, shoes, wheel cylinders, lines, rebuilt the front dual piston calipers, replace master cylinder and brake booster as well. Is there anything left????????

The only other thing that I can think of is vacuum. The 390 that is in it is Built. It has a very large cam in it. when I tuned the carb, I'm only getting about 12'' Hg. Is this enough?

Any Ideas?

[1999tjsport]

12 in sounds low, might be enough, how did you tune your carb? it can effect vac pres a lot, ign timing too, i don't know if their is really a good number that a brake booster wont work at

[steveg0690]

how did you tune your carb?

The carb and timing are both tuned to give me the highest amount of intake vacuum. I hooked a vac gauge up to the manifold when I tuned them. I'm thinking about trying a 12v accessory pump but I'm not sure yet.

[1999tjsport]

are you certain cam timing is good thats bit me in the ass and caused low vac, ive never used a acc pump before

[DixieJeeper]

12 to 13 hg/in is on the low side of acceptable- depending on your particular setup. With a hot cam and engine mods- I am not suprised its on the low side. I like to see in upwards of 17 hg/in which would be plenty for power brakes. Did you have good brakes before with the engine modfications? If so then 12-13 should be ok.

If you are equating it to "like having manual brakes again" I am assuming you have a high and hard brake pedal with increased effort to stop.

Double check the small stuff:

1)Is the vac hose from the intake routed correctly? It should NOT be 'T' anywhere and should be straight unported vaccum- straight off the intake manifold

2)Is the hose in good condition? Dry Rot? knicks... If you have a hot cam and she pulls a bunch more vac. on deaccel- if the hose is soft it can collapse inward on itself.

3) Inspect the check valve and valve and gromet. Obvisously, no cracks in either, and the valve should flow in one direction only.

4) Hard pedals are not caused by air in the system- but can be from restrictions- rebled the system check for a bleeder not putting out volume like it should- fault trace that line of the circuit back to the MC and look for abnormalities.

Baring that and assuming everything you have done has been rechecked and is good and/or the correct part (incorrect MCs can do what your describing) it is either:

A)Pushrod adjustment in the booster or pedal assembly

B)sticking proportioning/combination valve- I have seen them gum up from lack of brake fluid maintence and overheat conditions. Causing them to partially stick and give both hard and mushy pedals alike.

c)a collapsed line/hose which can give a loss of braking or hard pedal depending on where the resitrction is and how bad. Hoses sometimes can be intermitent and only collapse when hot or stressed. Look for bulges and defomations on the hoses. A collapsed line is easy.. look for a line thats crushed or pinched - if you wheel it pay attention to impact marks.

I hope that helps- I might have forgotten something- it is midnight afterall. Keep us posted.

[DixieJeeper]

If you determine that vaccum is the issue and its supply go for the pump you described and a vaccum canister too.. that way you have plenty of suction... (insert joke here)

[steveg0690]

Yes, I have a hard pedal. It actually seems a little low to me, not high.

I have checked all vac lines. The booster is new and is not leaking.

This is the second MC, the first new one that they sent me was incorrect, it was for all drum brakes and only had a small valve to push fluid to the front wheels. This one has the large bowl and valve.

I have bled the system a thousand and 1 times, I do not believe that there is air in the lines, all lines and hoses are in good shape.

I will check the push-rod. I forgot about that little sucker. I'm pretty sure I set it up correctly when I installed it, but who knows.

Thanks for the input. Not bad for after midnight.

[steveg0690]

If you determine that vaccum is the issue and its supply go for the pump you described and a vaccum canister too.. that way you have plenty of suction... (It's not my pump I swear! *Austin Powers* )

Inserted.

[DixieJeeper]

lol.. too funny... did you swap brakes? As in converting it to front disc? Perhaps if you did it is an incorrect prop/combo valve.

If you have a hieght sensing valve (some pickups have them to sense the load in the bed and increase rear braking force) check to make sure it is not sticking- its usually mounted under the bed above the axle on a cross member. If that is sticking it could be sending too much fluid to the drums. Another thing is to make sure you have the right wheel cylinders they along with the MC (sometimes only one of them) have residual pressure check valves. The drums need to retain a specific amount of fluid to react quickly when the pedal is pressed. Again an incorrect part might have the wrong residual pressure rating.

Did this problem just appear? As in you didn't have any problems with the current setup (no recent mods)- then wham one day brake issues? Or is this new after doing a specific repair/modifcation

[steveg0690]

Checked some stuff out today, still no change. MC push rod is correct.

This has been like this since I got the truck. At first I thought it was just from not being maintained. So I replaced pretty much everything. This truck came stock with the disc brakes in the front and drums in the rear. I've done my homework and all the parts should be correct. I have not checked the proportioning valve yet, but I've never really had a problem with one of them yet that would make me think it is the problem.

I'm not able to "lock up" the brakes at all. Which surprises me that the rear drums won't even lock up. This is why I have been leaning towards a vacuum assist problem.

[DixieJeeper]

Regardless of power/boost issues (as in lack of power assist) you should still be able to lock the brakes up. It might be harder- but it should still happen. That being said it sounds more like to me a fluid/pressure issue-

Which brings me back to prop. valve or MC/fluid delievery issue

[Sloth_Fratelli]

hydroboost and fuhgeddaboudit

[chux72vette]

I had the exact same problem on a '88 F-150 and after about $600 bucks, it turned out to be the proportioning valve. It was described to me that Yes, the breaks will lock up but Rear 1st as thats how the MC works, BTW. So in all intents and purposes, if A is malfunctioning we will never get to operate B, and so forth. I'm more of a Jeep/Chevy guy but brakes are about the same no matter make/model. My .02. Just hope ya get it and figure it out.~

[ridgerunner97]

hydroboost and fuhgeddaboudit

Hydroboost sucks. Blow the line and lose the brakes. Ask me how I know, had 20k on that was a fuggin treat **** hydroboost

[brooks294x4]

how about udgrading to a F350 DRW master cylinder? do a little research on the bore sizes and get the biggest possible.

my rig has discs all the way around, the 14 bolt had them when i got it, and it now has a 60 up front and still stops well with the 1/2 ton master and stock prop. valve. im going to put a 1 ton MC on in the future when i get some time.

i would try to bypass your prop. valve first and try again, or take it out and clean it. they are known to stick when they get old.

[ridgerunner97]

I'm currently chasing a brake problem as well, and think my prop valve is sticking, my XJ has the same exact symptoms as your Ford

[Cody Texan]

What kind of parts did you put in it?

Did you go with stock parts or did you modify it?

70's Ford brakes had a lot to be desired, but luckily you can remedy the issue by going to 1 ton brake parts.

The bronco locks up all 4 with no issue by following this build,
http://www.okcnetworks.com/bronco/brakes/index.htm

Also to get a proportional valve I got one from BroncoGraveyard.com

[steveg0690]

What kind of parts did you put in it?

Did you go with stock parts or did you modify it?

70's Ford brakes had a lot to be desired, but luckily you can remedy the issue by going to 1 ton brake parts.

The bronco locks up all 4 with no issue by following this build,
http://www.okcnetworks.com/bronco/brakes/index.htm

Also to get a proportional valve I got one from BroncoGraveyard.com

These are all stock parts. They should be more than suitable. My buddy has a 79 F250 that will put you through the windshield with stock brakes.

I think the next step is going to be pulling the Prop Valve and making sure that it isn't plugged.

I'll keep you posted.

[DixieJeeper]

Like I said- prop valve/combination valve should be your next move. You can convert and upgrade stuff all day long but if the basics aren't working you will get the same result.