Brake Booster questions

[Pebcak]

I've noticed lately that my brakes are "hissing" when I'm not applying pressure.  When I press the pedal at all it'll stop.  Since I replaced the Booster/Master Cylinder about 2 years ago I'm assuming that my Booster is having issues and needs to be replaced "again."  Sound reasonable?

After looking around in the engine bay and down by the pedals with the engine off/on, I didn't see or hear anything out of the ordinary except the hissing noise coming from the Booster area.  I checked all vacuum lines and nothing.  Even sprayed a little carb cleaner around the hoses to see if the engine picked up a little and still nothing.

With doing some searching and reading about Hydroboost conversions vs. staying stock I came across a post where a guy replaced his F150 Booster with a F350 Booster.  The post was written well and he said it was a 100% bolt on with no modifications.  

My question is, since I'm holding off on any BIG changes before I look into a rebuilt motor and trans, does it sound too crazy to attempt this?

Questions, comments or concerns are welcome as usual!

Thanks,

[Gary Lewis]

First, if you hear constant hissing when not applying the brakes then the booster must be leaking.  You can check that by using a vacuum source, like a Mityvac, to pull a vacuum and see if it stays or leaks off.  And you might even be able to determine if it is leaking by sucking on it, placing your tongue over the end of the hose, and going again and again.  If it isn't leaking you should be able to pull a vacuum on it.  If it is leaking you won't.

Assuming the F350 booster is vacuum-operated, what is the advantage of it over the F150 version?  Potentially it'll be bigger and give more boost due to the larger piston the vacuum is working against.  But do you need more boost?  In other words, do the brakes require a hard push to stop - like Big Blue's do.  And, will it last longer than the 2 years this one did?

[Pebcak]

Yes.  When the truck is just sitting and no brakes are being applied it will hiss.  As soon as I apply any pressure to the peddle it'll stop.  I still have brakes but they're a little spongy.  It's reminding me of the two"ish" years ago the booster went 100% bad and I coasted down on the back road and applied the parking brake to stop.  Thankfully I was on a back road instead of main road.

With asking about the bigger booster, I didn't know if it'll be heavier duty, last longer and/or give me a little more whoa if I wanted it.  Saw the post so I thought I'd ask "if" it's a possibility.  Or if I should go with another F150 booster.  That's no problem either.  For the difference of less than $10 I figured it's worth asking the question. :)

[Gary Lewis]

It is worth a try since you've seen a how-to that proves it'll work.  Let us know, please.

[Ford F834]

The “deep” F250/F350 booster has a different brake pedal than the “flat” F150 booster. The pedal pin is mounted higher. If you switch booster, you will need the right pedal. Most people know about this when converting to hydroboost, but there are different vacuum pedals as well. Not a big deal since it shouldn’t be hard to find, but something to be aware of.

[Ford F834]

Oh wait... and you are going to need the F250/350 master cylinder and brake lines as well. The F250/350 steel MC can mount to a hydroboost booster, but my F150 MC would not because the mounting holes were closer together. This means the F150 MC won’t fit the F250/350 vacuum booster either. If you buy a F250/350 MC to go with the booster, the threaded fittings are different than F150. The F150 has one large diameter fitting and one small. I looked everywhere for a reducer but no dice. I had to steal the one off my F350. Again, not a big deal as these are all common parts but you can’t just use the F350 booster with what you have.

For what it’s worth, my F350 vacuum brakes were horrible. They are being swapped to hydroboost. I don’t know if that booster has more ooomph than the flat one, but I was not impressed.

[Pebcak]

I went back and re-read the post.  It may have been a 1 for 1 swap because the vehicle in this post was a "89 F-150 5.0l"  I didn't think to look at that.  This may not be compatible with my 86 F-150 5.8l.

I might have the parts place pull out the two Boosters so I can compare and do some measurements.

[Gary Lewis]

I’m glad Jonathan replied. I didn’t realize that the pedal is different as well.  So, even if the parts look to fit, inc the master cylinder, you’ll need the pedal. Good catch.

[85lebaront2]

I put a 1990 booster and 1993 M/C on Darth, but I also changed the pedal to the 1992 up mounting and probably used the 1990 pedal on Darth.

[ArdWrknTrk]

IIRC the 150 M/C has  smaller bore than the 250/350 master.

Increasing the bore displaces more fluid per pedal stroke but also increases the pedal effort for a given line pressure.

Bigger trucks have larger caliper pistons that need more fluid volume and achieve a correct ratio with the bigger master cylinder.

[Gary Lewis]

Jim - With the larger booster does the 250/350 master/booster combo wind up giving the same pressure for the same pedal force?

[ArdWrknTrk]

Gary,
I don't know the details of the boosters, just that the bores of the master cylinder are different.
15/16" v 1 1/8" I think???

The mechanics of fluid displacement in a closed hydraulic system are simple ratios.

TIL that the pedals have a different pivot (changing the effort applied to the M/C by the operator)
Prior I only knew of this difference with the Hydroboost pedal.

If Ford changed the bolt pattern and the line fittings of the master it was to prevent the wrong one from being installed and likely causing injury.

[Gary Lewis]

I understand the physics involved with the bore sizes of the master and slaves as well as the leverage change induced by moving the pivot point.  But I don't understand the differences between the F150 and F250/350 boosters.  However, they are obviously matched to the master cylinder and should be kept together.

And, you have a good point about why Ford changed the bolt pattern and fittings.  Which begs the question "Why attempt this change?", which brings us back to the questions of "Has the booster failed?" and "Why did it fail in just 2 years?".

I found this explanation of how the boosters work.  And, within it says:

One quick test for leakage, is to turn the engine off and press the brake pedal. If the pedal still has one or two assisted applications before getting hard to press, likely no leak exists.

And then it offers this regarding why boosters fail:

When brake boosters fail the reason is often outside the booster itself.  For instance a bad master cylinder leaks fluid into the booster. Brake fluid will cause deterioration of the diaphragm resulting in failure. If misdiagnosed, the failed booster can provide a vacuum to the rear of a replacement master cylinder and quickly ruin it, repeating the cycle.

[Pebcak]

As usual, all of you bring up good points.  This is why Google is a dangerous thing!

I've checked everything along the engine bay for air leaks, discoloration and missing fluid.  I haven't seen anything. When I popped off the top to the Master Cylinder it's still full and fluid is still clear"ish".  About 6 months ago I had to top off the front section a tad, but that's it.

It's all been stable and sturdy since I swapped the Master Cylinder and Booster a couple years ago at the same time.

I will stick with my original plan (Before Google) of replacing with the same booster.  While doing so I'm going to do a good look at all the parts and connections in case I missed something.

[Gary Lewis]

I don't think that missing fluid in the front section will find its way into the booster.  But it probably doesn't take much fluid leaking into the booster to kill the diaphragm, and you might not notice the loss from the rear section.  So check the booster to see if it has fluid in it when you replace it.

Maybe you just got a bad booster.  That seems to happen more and more often these days with new parts.