RPMs sometimes vary with brakes

[Jdmick]

After searching and reading a little, I am guessing that my brake booster is going bad, but I thought I would see what you guys thought.
Every now and then when I am braking, it gets really hard to press the brake pedal, and when I get stopped the engine wants to die. It isn’t every time, but it seems that the harder I brake, the more likely it is to happen. Yesterday I took off the vacuum hose from the booster, and my rpms at idle went up by at least 500, I didn’t look at the tach, so it might have been more.
Do you think it is the booster, or is there somewhere else I should look?

 

[D'Animal]

Sounds like the booster to me.



Unplug the booster and plug the hose and see if it runs the way it is suppose to.

 

[Jdmick]

Is there a way to rebuild it, or just head to the parts wanted section?

 

[dgangle]

look for anything Toyota......most will bolt right on with a pushrod adjustment.

And do this soon or you are going to have a burnt #5 exhaust valve from the vac leak.

 

[handcannon]

look for anything Toyota......most will bolt right on with a pushrod adjustment.

If you go to a pic-n-pull be sure to get the plastic in line one way valve that goes with the booster. Most all the boosters you will find on mini trucks, etc will have this valve somewhere in the vacuum line. The cruisers valve is in the plastic inlet fitting that is on the booster. Also, some of the vehicles will have an aluminum spacer between the booster and the firewall. Get that with the booster and the one way valve.

Don

 

[Jdmick]

I took a closer look at the booster, and it looks as though it is fairly new. Is it possible that just the one way valve could be bad, or is there something internal that has probably gone out?

 

[dgangle]

the check vale only keeps vac on the booster for 1 or 2 pumps after the engine dies so one can still stop with less effort than with no vacuum. It doesn't matter what it looks like on the outside, it matters what condition's the inside.

Usually these leaks are brought about by a leaky MC into the booster somewhere in the trucks recent past. Pull ~20"wc vac on it and see if it holds.

 

[Magnum44]

The check valve prevents gas fumes from entering the diaphragm area on the booster when you shut the engine down. It also allows for a couple of brake applications if the engine dies. A bad check valve will allow the fumes to eventually damage the rubber diaphragm. You may have a leak in the diaphragm. When you apply the brakes a port opens to the atmosphere and this puts pressure on the diaphragm towards the vacuum side and if there is a hole or leak it will cause your engine to run lean and idle rough etc. It could be checked with a vacuum gauge but that sounds like the problem.

 

[Jdmick]

The check valve prevents gas fumes from entering the diaphragm area on the booster when you shut the engine down. It also allows for a couple of brake applications if the engine dies. A bad check valve will allow the fumes to eventually damage the rubber diaphragm. You may have a leak in the diaphragm. When you apply the brakes a port opens to the atmosphere and this puts pressure on the diaphragm towards the vacuum side and if there is a hole or leak it will cause your engine to run lean and idle rough etc. It could be checked with a vacuum gauge but that sounds like the problem.

If I checked with a vacuum gauge, and it tested okay, what else could it be?

 

[dgangle]

If I checked with a vacuum gauge, and it tested okay, what else could it be?

So did you check it or not and how did you do it?

Could be the vac line between the booster and intake.

Put a T in the line with a gauge. Start the engine, read/record the vac, turn off and see if the vac drop and if so how fast.

 

[Jdmick]

No, I didn't, I don't have a vacuume guage. I will probably go pick one up at test it. I took the master cylinder off and the check valve, and everything was dry and clean. I know that doesn't tell me what the inside looks like though. Just trying to avoid spending too much money if I don't need to.

 

[Magnum44]

I may have to think this through after I write this but if there is a hole in the diaphragm you may get vacuum until you apply the pedal. This is when the atmospheric port is opened and will allow air into the vacuum side and cause rough idle and of course no vacuum assist just a hard pedal. As long as you don't open the atmospheric port the idle is fine and the engine will pull plenty of vacuum.

 

[Jdmick]

I picked up a vacuum gauge, and tried it out last night. And I believe I did it correctly, this is what I came up with. I ran the test as dgangle recommended, I put a T in the line between the booster and intake. The first test the needs acted as though it was on crack, and fluctuated between 0 and 15. And if I am reading the instructios with the meter correctly, this means there is a leak. So I replaced the vacuum line to the booster, and the line from the distributor to cab for good measure. I ran the test again, and it still fluctuated, but it stayed between 0 and 5.

Could this still mean there is a leak in the booster, or should I keep looking? Or am I completely off base in my assumptions?

 

[Magnum44]

If I remember correctly you need close to 20 inches of vacuum to operate the booster properly. If the booster is leaking you won't get 20". Just for the heck of it take a vacuum reading off of your manifold without the booster and see what you get. Then if you have access to a vacuum tester (like a mityvac tester) apply vacuum to the booster itself and that will tell you a couple of things. One of which will be if there is a leak in the booster. If you changed the vacuum lines it's probably the booster. Some auto parts can loan or rent a vacuum tester and some can even test the booster. There are companies that specialize in rebuilding them too. Power Brakes Sales in Sacramento is a rebuilder and there are probably others around the country but maybe not in your area I don't know. the booster is where I'd put my money (if I had any).

 

[dgangle]

vac gauge tuning

How to Use and Interpret a Vacuum Gauge

 

[joeyg1973]

A quick and dirty way to find a vacuum leak is to spray starting fluid around the suspected area. Not sure it would work on the brake booster but it might be worth a try. Get someone to stomp on the brakes and spray the booster where you suspect the leak is. If the engine runs smooth suddenly you have located the leak. Like I said quick and dirty, but effective.

 

[dgangle]

Sounds liek a bad booster to me. Did you happen to run the seat-of-the-pants test? http://coolfj40.stores.yahoo.net/brakbooswor.html

To isolate the booster as the culprit, disconnect and plug the vacuum line from the intake to the booster. If vacuum reading/idle stabilize/change significantly/remedy you have just found your problem. Replace with anything Toyota...Celica, mintruck, 4Runner, Supra, 60 series, 80 series, Lexus LX450...................

I have never been successful at using a manual Mitivac to pull enough vacuum on a booster to determine if it's any good. Too many leaks/pumps to be a realistic means. Nor at using an accelerant into the booster. It's one thing to spray it around the base of a carb and an entirely different thing to attempt to spray it into a an enclosed, confined space.

 

[Magnum44]

A quick and dirty way to find a vacuum leak is to spray starting fluid around the suspected area. Not sure it would work on the brake booster but it might be worth a try. Get someone to stomp on the brakes and spray the booster where you suspect the leak is. If the engine runs smooth suddenly you have located the leak. Like I said quick and dirty, but effective.

That's a good way to check for leaks around the engine compartment but the diaphragm in the booster might not like the starting fluid. It's susceptible to gas fumes and starting fluid may have the same effect.

 

[Jdmick]

Another question, could the loss of vacuum be caused by my carb being out of tune? The reason for this question is that I made some adjustments over the weekend, and since the changes I didn’t have any issues on the drive to work this morning. Here is what I did. Evidently the PO used the wrong grommets for the breather, and pcv valve, it was blowing oil out of the breather. I hooked it up correctly, and so far, no oil coming out. I also changed the step up springs. And after reading the manual on the carb closer, it recommended that the fuel pressure shouldn’t exceed 6.5 lbs., and my pump was putting out 10.5 lbs., so I put in a regulator and dropped the pressure to around 5 lbs.

After all of this, it seemed better. But could it have actually fixed my issues?

 

[Jdmick]

I replaced the booster, and the problem remains. Granted it was a used booster, and there is no guarantee it was good either. But the brakes to feel more solid, with the old booster the pedal would go all the way to the floor to, now I only need to go about half way.

Should I send one of them off to get rebuilt, that way I know it is good?