Brake Booster
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Joined: Dec 2009
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Brake Booster
I just bought an 85 f150 with a 300 6 a few weeks ago and it makes a hissing sound whenever the brake isn't pressed all the way. The guy I bought it from said that it was a leak in the brake booster but it seems to work fine so I was planning on not worrying about it for a while. Currently, the truck gets right around 9-10 mpg. When i'm at a stoplight and I press the brake all the way so that the hissing goes away I can hear the car idle lower.
Do you think that this could be affecting my mileage that much or should I be looking elswhere for the problem?
I know these trucks don't get great mileage, but from what I'm reading most people aren't doing this poorly.
Also, from what I can tell everything else seems to be in good order except that there's no exhaust past the header.
Do you think that this could be affecting my mileage that much or should I be looking elswhere for the problem?
I know these trucks don't get great mileage, but from what I'm reading most people aren't doing this poorly.
Also, from what I can tell everything else seems to be in good order except that there's no exhaust past the header.
More Turbo
Joined: Sep 2008
Posts: 591
Likes: 54
From: Pennsylvania
I had the same problem, and I also noticed the engine performance taking a hit. My carb's not so good as it is, so it would take even longer to start and it had a tendency to shut off if the brakes were pressed if the engine wasn't warm enough. I wouldn't be surprised if it was hurting your gas mileage, old engines aren't great on the MPGs but a 300 should still be getting more than 9-10.
ALSO, just like you, I let mine go for a while, and eventually got around to putting a new one in with no issues. BUT! Just a word to the wise, a broken brake booster could go out at -ANY TIME- and you could lose power brakes, so be cautious. The sooner you can get a new one in there, the better.
ALSO, just like you, I let mine go for a while, and eventually got around to putting a new one in with no issues. BUT! Just a word to the wise, a broken brake booster could go out at -ANY TIME- and you could lose power brakes, so be cautious. The sooner you can get a new one in there, the better.
Fleet Mechanic
Joined: May 2009
Posts: 1,460
Likes: 6
From: Bay Area, Unfortunately
I thought someone posted on here a link to one of the chain stores that it was actually CHEAPER to get a remanned booster and MC than to get the booster alone. If that is the case, now is a good time to replace both for good measure.
pedant
Joined: Jan 2008
Posts: 23,576
Likes: 40
From: EXTREME southwest CT
Be mindful that it's often a leaky rear M/C seal that causes the booster to fail.
If you find that there's brake fluid present when you unbolt the master from the booster you will need to replace it too.
Bench bleed the unit before installation.
If you find that there's brake fluid present when you unbolt the master from the booster you will need to replace it too.
Bench bleed the unit before installation.
pedant
Joined: Jan 2008
Posts: 23,576
Likes: 40
From: EXTREME southwest CT
If it is air bound it will not pump any fluid at all.
Much easier to bleed it while you are able to tip it to all angles than while bolted in place.
I don't think anything "bad" will happen, but it can't be good to run the piston seals up and down the bore dry.
Much easier to bleed it while you are able to tip it to all angles than while bolted in place.
I don't think anything "bad" will happen, but it can't be good to run the piston seals up and down the bore dry.
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Fleet Mechanic
Joined: Apr 2007
Posts: 1,429
Likes: 4
From: patricia ab canada
i have seen the rubber o-rings fail from not bench bleeding.out of the hundreds i have installed this happened to 3 or 4. the bore is dry and the o-rings stick to the bore,the brake fluid lubes the o-rings as well.i just fill the master cylinder with brake fluid and use a vacuum pump and use it to draw the fluid into the the bore. or you can mount it in a vice,fill with brake fluid and push the plunger(i use a 3/8 socket extension) in until you get fluid out of the brake line holes.
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just wondering cause the other day I was changing my master cylinder and I was in a hurry (and stuck in a parking lot till I got er gone
) and I didn't bench bleed the cylinder. I just bolted it on, put fluid in, and proceeded to bleed the brakes. I had to go around a couple times and a lot of air came out of the front 2 bleeder valves but the pedal feels just fine, nice and hard, not spongy at all. That was one of the few more involved fixes I've ever done on that truck where (almost) everything went without a hitch and it actually worked out just right. 
) and I didn't bench bleed the cylinder. I just bolted it on, put fluid in, and proceeded to bleed the brakes. I had to go around a couple times and a lot of air came out of the front 2 bleeder valves but the pedal feels just fine, nice and hard, not spongy at all. That was one of the few more involved fixes I've ever done on that truck where (almost) everything went without a hitch and it actually worked out just right.
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