Working on my 1970 F250 (this one) and the brake master cylinder apparently leaked into the brake booster when I replaced it a few years ago. I had the master cylinder off and took the booster off to paint it (damage to the paint from the previously mentioned leaking m/c) and it drooled a ton of brake fluid out. Racking my brains, I realized that means it sat for like three years full of brake fluid. It still works but is it going to keep working? While I have it out, should I just go ahead and replace it? My main hesitation is that modern parts seem to be really low quality for these vehicles but I'm still thinking that rolling the dice on a reman might be better than rolling the dice on a brake fluid soaked rubber membrane that's at least 15 years old.
Any strong opinions?
Run it. They don't tend to suffer too much from brake fluid as you might think they would.
Worst case, you get a vacuum leak in the future and perhaps a loss of power assist and you wind up putting a pile of junk reman on it.
A working factory part is always better than a parts store reman in this day and age.
I wonder what kind of rubber it is, and whether it's deteriorated by brake fluid...
Forever failing to keep these things straight, I found this primer on o-ring/seal compatibilities at Pegasus the other week, and found at least one reference to the diaphragm being EPDM in a quick search...
My first thought, though I don't wish to be cavalier with brake bits, is that the booster probably won't go suddenly, won't take total braking ability away when it goes, and may be just fine. If it weren't for the dodginess of new bits, I'd probably lean toward replacing it. I'm guessing on a '70 F250 it's not much trouble to replace it after the fact (save for the intrinsic pain in the butt that is bleeding).
OTOH, if that's the original booster, it's 55 years old. Brake fluid degradation or no, that's a lot of sitting, exposure to... air, and flexing. I also wonder if there are better options than FLAPS bulk remans... Anybody know anything about https://www.applehydraulicsonline.com/collections/brake-booster-rebuilt (they seem to offer, possibly depending on application, services and kits)
I'd replace it. The brake booster gets air in from the cabin when you apply the brake, then it gets exhaled into the front of the booster when you release the brake, then the air goes into the engine via the hose. So there's always going to be airflow through it.
Air contains moisture, and the vacuum with the engine running isn't enough to boil all of the moisture out of the brake fluid, and it can freeze, assuming it doesn't damage the bellows first either from chemical interaction or rust on the reinforcing plate wearing a hole in the diaphragm. Where did the rust come from? The waterlogged brake fluid...
I had a booster freeze on me once, that was fun. Fortunately I hit a concrete wall and not a car or a person, and it was slow enough that it just scuffed the chrome on the bumper.
SOP anywhere I worked was if there was evidence that brake fluid got into the booster, replace the booster. Braking isn't any place to say "it'll probably be fine".
If the booster is rusty outside, like pitted and not just rust coloring, I'd replace it too. I've seen boosters rust out from the outside in.
Pete is right, but my cheap ass would send it.
I've run boosters that have had brake fluid leak into them, but only in a hot climate, if it sees subzero temperatures that freezing problem is too scary to risk...
I just had a thought. My mindset has been for modern boosters where the master end is open to the master.
You do not have this. Your booster should be sealed to the actuator pushrod. You should be able to run the engine with the master off and have no vacuum leak.
If there was fluid in it, -the booster is already bad- because it is no longer sealed to vacuum. Furthermore, this vacuum leak is highly probable to have damaged the master cylinder, which is not designed to have a vacuum applied to it.
Regardless of theories about fluid damage, the presence of fluid is proof that the booster is kaput. It should not have been able to get in there in the first place.
In reply to Pete. (l33t FS) :
Excellent point. I wonder if my multiple failed master cylinders is due to that. I've had two go bad pretty quickly and blamed it on cheap parts but maybe it's my booster.
If multiple masters failed, it was definitely the booster.
30 years ago it was SOP to check for vacuum with the master off if replacing one that was leaking externally from the piston. That is institutional knowledge that has slipped away now that the designs are different.
I remember 18 year old Pete being told that. And the warning to not touch the brake pedal with the master off.
