1987 6.9 F350 crew cab hard stoping Brakes

[1Turbo10]

Hi All,
I am a new member and have enjoyed learing all about my new toy. a 1987 6.9 crew cab T-19 4.10 DRW. My problem is the way it stops. The Truck now has new pads, rear shoes, wheel cylinders, Axle seales were bad so I changed them too. I have also replaced the vacume pump with oem and the brake booster Adjusted and fulley bled. I will tell you I own a race shop and this stuff should be no big deal. I may just be to fussy but this thing is just not a great stoper. The pedal is high and firm and stops well under normal conditions, problem is when you stop hard the pedal seems to hit a hard spot and the stoping power goes away and you have to push like hell. I will be pulling a 30ft trailer and think they should perfome better. You guys seem to know these trucks better than anyone. Is this normal?
Is the new vacume booster defective? Thanks in advance!
John

 

[JPR]

When you apply the brakes hard while stopped and hold it, does the pedal travel a bit more after 10 or 15 seconds? If the vacuum pump capacity is low, the vacuum booster will travel a shorter distance before vacuum dips. Even when the pump and vacuum canister are fine, there is a longer recovery time with the pump than with a 460 cu in engine creating the vacuum.

Since you are towing that large of trailer I would consider the hydroboost conversion.

 

[Full Monte]

You are experiencing the same problem that many here have had. I have an 85 that wouldn't stop well until I put on hydroboost. See the article in the tech articles section. See also my truck in the signature. They changed the diameter of the vacuum assist unit somewhere in the '87 timeframe to increase stopping power. If you adjust the rears to have almost no free play, you may improve things without going hydroboost. Hydroboost doubles the pressure in the brake lines at the wheel cylinders.

 

[icanfixall]

In your post you said the peddle is high... Thats a good thing but... Set the parking brake. If it sets more than half way down then your rear shoes are way out of adjustment. Non of these trucks rear automatic adjusters work very well. We usually manually adjust them when the brake peddle sinks after we stop or the parking brake is set more than half way to the floor. When the rears are out of adjustment the master cylinder can't push enough brake fluid back there to make them work. Then the fronts do all the stopping and they will overheat and crack the rotors. Happened to me twice till I learned. The rear shoes are properly set when the shoes are around 25 thousands from the drums. Thats about impossible to measure but thats what you need to do. The suggestion about changing to the hydroboost brakesis something I personally did. Use the brake peddle from a donor truck and the 1 5/16 inch master cylinder and you will have the best brakes ever. I was lucky. I found an F450 in a wrecking yard with a new master cylinder on it. The brake peddle pin is at a differant height of 1 inch so just take that off the donor truck. Its really easy to do...

 

[Agnem]

....Non of these trucks rear automatic adjusters work very well. ...

The adjusters usually seize up from lack of maintanance. If you keep them lubed, they work fine.

 

[Optikalillushun]

i have to disagree, i cleaned mine on my half ton and they didnt work right.

 

[RLDSL]

Brakes being not enough on these things is subjective. If you're used to passenger cars, race cars, especially european cars...they suck. If you're used to driving overloaded semi trucks full of cut chicken through the Ozarks with chicken fat drippin's soaking the brakes... the brakes on these things are downright great
If I had an autobox I'd probably want the hydroboost, but with the zf5, Mine is quite comfortable yanking a 10k 5er around in the mountains, then again, I'm not trying to compare it to a sports car.

 

[1Turbo10]

Thanks guys,
Sounds like the Hydra boost system is the way to go. Wish I knew that before installing the new vacume booster. The rears are all new everything adjusted a couple of times and bled, pedal is great till I hit that hard spot. Master holds great no bypass. One of you mentioned a bigger booster. Funny there were two choices when they delivered the one I have and mine was the smaller one! Can I install the bigger one? Has anyone tryed it?

 

[towcat]

Thanks guys,
Sounds like the Hydra boost system is the way to go. Wish I knew that before installing the new vacume booster. The rears are all new everything adjusted a couple of times and bled, pedal is great till I hit that hard spot. Master holds great no bypass. One of you mentioned a bigger booster. Funny there were two choices when they delivered the one I have and mine was the smaller one! Can I install the bigger one? Has anyone tryed it?

you will either get one with a single lung or a dual lung. Either way, they pale in comparison to the hydro.
Out of curiousity how old is your proportioning valve?

 

[icanfixall]

Yes, the larger master cylinders will fit with some rat tail file work on the mounting holes. They need to be opened up just a little.

 

[1Turbo10]

P Valve is origional I assume. I was asking about the vacume booster but now that you mention it is there a bigger master cylinder with more pressure or volumne. Do you use the f450?

 

[frdtrckmnrich]

Does your truck have the weight proportioning valve for the rear brakes?

 

[Mr_Roboto]

P Valve is origional I assume. I was asking about the vacume booster but now that you mention it is there a bigger master cylinder with more pressure or volumne. Do you use the f450?

A larger master cylinder will make the brakes harder to press, not easier. (The master will displace the same volume, with less pedal travel, equalling a reduction in mechanical leverage).

 

[1Turbo10]

The proportioning valve is origional I assume. Is there someting I should check on it? Just a thought, this truck has an old trailer control unit tied into the master, could that be sucking up the volume I need? I think its tied into the front which is the rear brakes. Should it be tied into the fronts or just trash it for a new style?

 

[JPR]

Does your truck have the weight proportioning valve for the rear brakes?

good point, had forgotten that. Check for a valve mounted to the frame above the axle. There should be a two piece arm assembly that connects the rear axle. On my 86, there was stupid plastic grommet that would allow the arm to pop loose which would cause the valve to think that the truck was unloaded and reduce the rear braking.