Brake Vaccum Question

[aiyana7.3]

So I'm new to this site, but I would like to thank everyone for helping me out so much in the past!

I have a 1992 7.3l idi Ford f250 Heavy Duty XLT. It has new calipers, rotors, and pads on the front. I have a spongy brake pedal, so first I bled the brakes, then the pedal would build up pressure but slowly lose it. Then I put a new master cylinder in and bled the brakes again. Now, with the truck off it builds up pressure, but as soon as I turn the truck back on it loses pressure and they become spongy again.

I think that it might be the vacuum pump, I have a vacuum gauge coming in tomorrow from auto zone. When I disconnect the line that leads to the vacuum pump I feel suction on my thumb but the brakes are really firm and stay pressurized when the vacuum pump is disconnected.

I've been told that idi's usually have a spongy brake pedal but right now I can push the pedal halfway before the brakes engage. Is the vacuum pump bad, is it something else that I haven't thought of yet, or are they normally that way?

Thanks everyone for all the help.

 

[rattleonby]

Ok. First welcome!! Please check out hall of shame for shady and shoddy workmanship and quality sadly performed by more shops than we'd like to have on this planet. Hate seein guys get burned, we all work very hard for our money.Ok now the goods...

With the master and pretty much the front braking system replaced and you said you still have a "spongy" pedal. Question is with the truck running and sitting still take time to see if your pedal pumps up. Dont do fast crazy hard pumps, just normal semi-firm pumps spaced far enough apart to take notice where the pedal finally gains pressure.

If it does pump up yes you still have air. If it does not, take notice to see if you have a leaking wheel cylinder and also adjust up your shoes.A little bit of drag is always acceptable in these old heavy pigs, not going to hurt em. this is usually done best with the rear wheels jacked, ON JACK STANDS, and the FRONT WHEELS CHOCKED! Ive seen too many guys cruch feet for this simple job. These trucks are heavy as hell.

Now as for a failing vacuum pump or a leaking booster you would experience a loss of power assist...or a hard or stiff brake pedal. now u can test vacuum off the pump all day, (good vacuum output at idle usually above 16-18hg but dont quote me on that but usually)

Now if the REAR brakes and booster and pump all check out and you adjusted your rear brakes as per said above then you have a couple of options guys on here have done...

: Go Hydroboost....Meaning a hydraulic power assist system using the power steering pump for the power assist source. Plenty of guys on here who have done this swap sourcing the parts from ford F-superdutys (early F-450's)...most commonly seen in roll-back or stake body form

:Rear disc brake swap also sourced from F-superdutys...would take care of any needed adjustments to the rear brakes and some of the low pedal sickness all OBS ford F2's and 3's have...diesel OR gas. Also plenty of swap threads and plenty of experience from members on this site who have made those threads.

:Live with it like i do. These trucks have always had this problem and i'm still running the stock vacuum system. Yes it is scary at times with 3000-4000 pounds in the bed like i sometimes have but its crazy what a grown man can get used to isnt it?

I hope my ranting has guided and sorted out some of your direction

Merlin

 

[aiyana7.3]

Thanks for the reply!

With the truck on I pressed the brake to the floor. The second time it had a little more pressure then the third time halfway down I feel pressure but I can push through it. From there on keeps the same amount of pressure but doesn't gain any... nothing like when it is off... when it is off it gains a lot of pressure and keeps it until you start it. Since it did pump up, does that mean that there is still air in the system and I should bleed them again? I bled it more than five times throughout this whole process but it hasn't changed the pressure at all.

How do you tell if you have a leaking brake cylinder? Can you tell me a little about adjusting the brake shoes? I've never dealt with brake drums before.

Thanks for all the help and sorry about all the questions!

 

[icanfixall]

Welcome to the forum. Your spongy brakes usually is from the rear shoes being way out of adjustment. To see if thats the issue just apply about 1/4 to 1/2 of the parking brake. Now test drive about slowly and see where the brake peddle stops the truck. I'm betting it stops the truck much higher up in the trvel now. That will prove if the rear shoes are too far out of adjustment and you need to manually adjust them with a brake spoon. Most of us know this and just do it. The self adjusting rear brakes never really did work on most of these trucks. Its a dangerous thing to because the front brakes are the only parts doing any braking and thats heating them up too hot when you have a load on the truck. This is alow why the brake peddle slowly drops to the floor and you roll forwards to the stop lite usually. Whats happening in the brake system is the peddle has filled the front calipers but the back wheel cylinders need more fluid to fill them. So the peddle falls and it acts like a bad leak or bad master cylinder. It can be confusing but its easily fixed by manual adjustment.

 

[OLDBULL8]

Lesson in how the brake vacuum booster works. With the engine off, you have no vacuum to assist in helping you push the brake peddle down, that makes a hard peddle, with the engine running you have vacuum, when you push the brake peddle down, the vacuum helps pulls the brake peddle down. With the engine off, push down as hard as you can, keep pushing, if the peddle goes to the floor,then you have a brake fluid leak someplace, if it don't go all the way, then you still have air in the brake system. The correct way to bleed is, right rear, left rear, right front, left front, keep plenty of fluid in the MC. If the emergency brake peddle goes down more than half way, then the rear brakes need adjusting. Jack the rear up so you can spin the wheel, on the inside of the brake drum at the bottom is a little rubber seal, pop it out, then with a brake adjusting tool, turn the star wheel inside there until you can't turn the wheel, then back it off until you can just turn the wheel and it rubs a little. turn the star wheel up to tighten.

 

[yARIC008]

Did you properly bench bleed your master cylinder? If not you may still have a bit of air left in the system. Also, if you have rear ABS, did you bleed the ABS module?

I had alot of the same problems you had but it turned out most of my problems were because I accidently took off the the metering valve on the master cylinder for the front brakes. Another thing I added was a residual valve on the rear brakes which seems to have helped my braking too and helps prevent the rear wheel cylinders from ever sucking in any air around their seals.

Really taking some good time and flushing ALOT of fluid through the brakes makes sure you have no air. I think I put about 2 gallons through mine before I was convinced I had no air in my lines. I put on those one way bleeder valve screws and that really sped up the process. Another thing to look at, were the front brake lines from the truck to the calipers replaced? Those rubber lines can go bad and sponge out like a balloon and or burst.

As for your question about how do you know if your rear wheel cylinders are leaking, well... basically just look at them. If you don't see any fluid on the bleeder screws or on the plungers when you take off the drum, then they're not leaking.

 

[aiyana7.3]

The e-brake doesn't work... it might work a little tiny bit but I can still roll down a slight hill with it fully engaged so I doubt it is working. It also goes all the way down to the floor easily, so it looks like I need to align the rear brakes... I'll try that tomorrow. The only thing I don't understand is how the brake pedal is supposed to work when the truck is off...

 

[yARIC008]

The e-brake doesn't work... it might work a little tiny bit but I can still roll down a slight hill with it fully engaged so I doubt it is working. It also goes all the way down to the floor easily, so it looks like I need to align the rear brakes... I'll try that tomorrow. The only thing I don't understand is how the brake pedal is supposed to work when the truck is off...

Sounds like def. adjusting your rear brakes will help you greatly. What do you mean you don't understand the pedal with the truck off? Basically without the booster helping you it requires more foot pressure to push the brake pads and shoes. All the booster does is give your leg more muscle.

 

[aiyana7.3]

With the engine off, push down as hard as you can, keep pushing, if the peddle goes to the floor,then you have a brake fluid leak someplace, if it don't go all the way, then you still have air in the brake system.

this is what I was referring to... if it goes to the floor its a leak, if it doesn't go all the way its air, so what is it normally supposed to do? Again, sorry about all the questions

 

[OLDBULL8]

Lesson in how the brake vacuum booster works. With the engine off, you have no vacuum to assist in helping you push the brake peddle down, that makes a hard peddle, with the engine running you have vacuum, when you push the brake peddle down, the vacuum helps pulls the brake peddle down.

You don't understand this? You need to get a couple of Haynes books a 10330 and a 36058.

 

[aiyana7.3]

No, that's the part I do understand... the only thing is what the normal behavior of the pedal when the truck is off is supposed to be... cuz if when it goes to the floor its a leak and if it doesn't then its air then whats normal

 

[yARIC008]

No, that's the part I do understand... the only thing is what the normal behavior of the pedal when the truck is off is supposed to be... cuz if when it goes to the floor its a leak and if it doesn't then its air then whats normal

If you have a leak the pedal will push to the floor. If you don't have a leak it will not push to the floor. I'm not 100% sure if i follow Oldbull there either on the air thing. If you have a tiny amount of air in the system the pedal will be slightly more "spongy" than if the air wasn't there because the air is compressible. Bottom line I think is, if you push the pedal hard and hold it and the pedal doesn't fall all the way down then you don't have a leak and you may or may not have air in the system.

 

[LCAM-01XA]

no air or leaks = you push on pedal and it goes down and then stops like you have a brick under it, you can damn near jump up and down on it (don't try that!) and it still will barely move.

air in the lines = you push on pedal and it goes down and stops kinda softly, if you try pumping it it will feel spongy as if you have a basket ball under it.

hydraulic leak = you push on pedal and it goes down and usually doesn't stop till it almost hits the floor, at this point there's a puddle of brake fluid forming somewhere under truck

 

[jay_t]

I had brake trouble, spongy pedal, build and lose pressure, fade at lights. Had complete new up front, lines, and rear complete new. Better, but still something not right. Couple weeks ago while driving down a back road the brakes were rock hard, No Power. Was in the car with my wife and did not want to deal with it because of the cold, so I just kept driving home. No mention. It's a manual and back roads no traffic, so I could stop but took a while. I found the alternator loosened up and threw the vacuum belt. I replaced and re-tightened. My brakes have never been better!!! I did need brakes all around, but this made all the difference in. My guess is the belt was failing for awhile.

 

[fsmyth]

I have a little different problem -- and it just started.
When I apply the brakes (which work fine), the pedal wants to stay down (and keep brakes applied).
Is there a spring on the pedal that may have broken? Or is there some valve in the booster that may have failed?
I'll get out and check it as soon as it warms up a bit - I'm getting too damn old to be crawling around in the snow.
TIA
<als>

should have added - I can pull the pedal back up from behind, and it stays up until next time.