Braking Stiff only when Applied Urgently

[MICHAEL MICHAUD]

Gentlemen,

I posted about this in my build thread but im just trying to tie off some loose ends before I head back home.

My brakes work fine consistently if I apply the brakes slowly or average coming to a casual or evn a little faster than casual stop, meaning i feel the brake assist working and I stop well.

But, If I slam the brakes or apply them urgently like you would if you noticed someone else in front of you applied their brakes unexpectedly the pedal stiffens up and my brake assist doesnt kick in and I come to a stop extremely slowly and sometimes I wonfer if Im going to have to veer off the road to prevent an accident. If i let off just a little and apply gentle pressure eventually the assist will kick in.

Now this isnt a huge problem If i just make sure to not stay up peoples ass. But obviously something isnt functioning correctly.

Any ideas? Im thinking the slam moves the diaphragm so quickly that somehow either its leaking at the check valve or from the back end, it cant be leaking all the time because it usually works fine.

Maybe the angle of the rod is messed up for the slam. Either way. I just bought a new Vacuum pump that I havent installed and id like to return if possible. Should I just swap out the booster?

The grommet and check valve seem to be sealed and funtioning properly. Also once I feel the brake assidt kick in and the pedal gets a little soft i can apply them as hard as I want and stop very quickly. Its like an assist delay basically and I have to wait for it to respond.

Let me know what you guys think.

Thank you!

Edit* I have tested the pressure the vac pump is putting out but not the volume. The vacuum hoses and vac system are not leaking.

If its time to replace the booster what brand have you guys been confident in. I think Duralast seems to be pretty decent compared to carquest or Advanced. Oreilly too I suppose. Maybe Napa is ok?

 

[YJMike92]

The check valve is there to help maintain the vacuum in the booster. If on a quick application the pedal is hard it would seem the vacuum would have drained out of the booster or never made it to the booster. A standard test for the booster on all vehicles is to start the car let it run to build vacuum. Shut the engine off and let it sit for several minutes. with the engine off you should be able to pump the pedal two or three times before the pedal becomes stiff again (which indicate your booster is capable of maintaining vacuum) If it were me I would make sure no restriction exists and the check valve is not sticking closed or open. Could something have gotten in the hose while the engine was out (dirt dobber)? If your vacuum pump is good hoses are not restricted valve is not stuck open or closed my guess would be a booster??? As far as brand of a booster all parts store remans scare me.

 

[MICHAEL MICHAUD]

The check valve is there to help maintain the vacuum in the booster. If on a quick application the pedal is hard it would seem the vacuum would have drained out of the booster or never made it to the booster. A standard test for the booster on all vehicles is to start the car let it run to build vacuum. Shut the engine off and let it sit for several minutes. with the engine off you should be able to pump the pedal two or three times before the pedal becomes stiff again (which indicate your booster is capable of maintaining vacuum) If it were me I would make sure no restriction exists and the check valve is not sticking closed or open. Could something have gotten in the hose while the engine was out (dirt dobber)? If your vacuum pump is good hoses are not restricted valve is not stuck open or closed my guess would be a booster??? As far as brand of a booster all parts store remans scare me.

Haha im eith you on the fright of the remans. Where can I get a good one?

 

[YJMike92]

Looks like the dealer can still get them.
This fits my truck, I don't know about yours.
https://www.fordpartsgiant.com/parts/ford-booster-asy-brake_f5tz-2005-ca.html

 

[Thewespaul]

It does sound like a booster issue to me. Do the test mike said, I’d hang onto the pump for now but replace the booster first. I’ve had no issues with o’rielly boosters, and they have a core charge that brings the cost down significantly.

 

[corrado9184]

i have this same issue! replaced everything! the master cylinder, booster, Vacuum pump, new check valve, directly plumbed it to the master cylinder with new line, replaced the front calipers, front brake lines, flushed the brake system and adjusted the rear brakes!
Only thing i haven't replaced is that damn proportioning valve, so that's my next step.

 

[MICHAEL MICHAUD]

Damn people have bet me money its the prop valve. Im literally on the way to the store to buy a puller that i can cut up.... maybe I should just put my vac pump back in.... I replaced the booster and I have the same exact problem. Its got to be some vacuum issue I think though. Im buying new hoses for the vac and booster too.When i press gently wleverything works i just dont understand this.

Thanks for the reply. Let me know how that goes for you!

My next step will be a vacuum gauge so i can see exactly what is happening at the booster when its misbehaving like an ungrateful jerk!

 

[BrianX128]

You could also have a small slow vacuum leak. When you press gradually at first the vacuum builds back up slowly and doesn't deplete to the point that the booster stops working.

Can you hook that vacuum gauge you have inline with your vacuum lines and watch it from inside the cab if you can wedge it under the hood somehow to where you can see how much vacuum you lose with various efforts of pressing the brakes down? When I had issues on my 6.9, I had a slow leak at the tree. It would build 28hg of vacuum, but if I used the brakes even some or slowly pressed and released it would drop to about 9. Slam them and it bottomed out to 0 and took 3-4 seconds to recover to 9, and probably ten seconds to get back in the 20's.

 

[jericho]

Check your ventilation control.It can leak in just certain positions. For example mine leaks only with the selector in the off position, but not heat, ac, defrost, etc. Turn the engine off, then listen in the cab for an air sound in the different air positions. Mine only bleeds off vacuum only at the off position. Tee in a vacuum guage to be sure.
Still, I suspect the proportioning valve.

 

[MICHAEL MICHAUD]

You could also have a small slow vacuum leak. When you press gradually at first the vacuum builds back up slowly and doesn't deplete to the point that the booster stops working.

Can you hook that vacuum gauge you have inline with your vacuum lines and watch it from inside the cab if you can wedge it under the hood somehow to where you can see how much vacuum you lose with various efforts of pressing the brakes down? When I had issues on my 6.9, I had a slow leak at the tree. It would build 28hg of vacuum, but if I used the brakes even some or slowly pressed and released it would drop to about 9. Slam them and it bottomed out to 0 and took 3-4 seconds to recover to 9, and probably ten seconds to get back in the 20's.

I couldnt get the pully off my old vacuum pump with the dismal tools available so I bough a new big vac hose and some worm hose clamps and put her straight to the booster again. I tested the brakes and they work alright, I just have to realize its not a sports car. I couldnt get them to be stiff without deliberately applying the brakes in a way to bleed vacuum like several back to back hard stops. So i think maybe there is a small leak in the hvac like he said in the off position or something. I know my tree holds pressure.

I still have my defroster if I need it and im happy eith that till I get home. I dont have time to be messing around with luxuries in my truck haha I have to be outta here by sunday night. Havent packed up the garage yet... oooof.

Thanks for all the help guys lots of good ideas here and places of past failures so Im glad I made a whole thread. Hopefully it will save me and someone else a few hundred bucks.

My trailer brakes are brand new. Brake controller is working. Just gotta pull her home.

 

[Macrobb]

Puling the vacuum pump pulley requires a special puller tool. Autozone rents it for like $40, return for full rebate.

Chances are, your vacuum pump is worn out - it's not pulling as strong of a vacuum as it should, nor as much CFM(total air flow). So you don't have the 'reserve' when you slam on the brakes.

You may be able to test this by keeping the engine revved up to like 2K while slamming on the brakes - if it works better, you know the vacuum pump needs replaced.

 

[MICHAEL MICHAUD]

Autozone has the tool for sure? I went to advanced vut they didnt have it.

 

[Macrobb]

Look for power steering pulley puller. It's the same thing.

Edit:

Another option to save time is just bring the new, old pump and a few tools with you. Just swap the pulleys in the parking lot, install it and done.

 

[chillman88]

Autozone has the tool for sure? I went to advanced vut they didnt have it.

Advance probably does but just doesn't know it. Like Macrobb said, look for the power steering pulley remover. I got mine from Advance so they should have one.

 

[BrianX128]

Advance probably does but just doesn't know it. Like Macrobb said, look for the power steering pulley remover. I got mine from Advance so they should have one.

Yeah I got mine at advance before too. It's a shame you can't buy our vacuum pump with the pulley on it. My 7.3 the pulley is bent and the pump is old as dirt, I figured I'd replace it over winter and was surprised you can't buy it that way.

Also its beating a dead horse with stupidity of the counter people at some of these stores but I stopped at oreileys tonight to get a 175amp mega fuse for my 3g swap and when I said that the guy literally went, "huhhhhh? - 175amp fuse for a vehicle?" Like I wanted the flux capacitor next.