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Old 11-13-2006, 01:48 PM   #1 (permalink)
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Brake Problem...master cylinder or booster? Not sure...

Came home this weekend from school to my '87 pickup. Noticed the brake pedal felt a little funny. As I drove it throughout the weekend it progressively got worse and now every once in a while the pedal will have more travel than usual. Like the pedal would go all the way to the floor before I get any brake...but, once I get brakes I can stop like normal, no trouble stopping or locking them up.

I thought maybe it was the booster, but since I have the stopping power I don't know about that. (I have tried stopping without the truck running, so I have an idea what the pedal will feel like and how hard you have to push it without the booster)

No signs of fluid leaking...though I haven't got a chance to really get down and investigate it.

Could it be the master cylinder giving me the troubles?

It is very sporadic...I mean as far as when the pedal will sometimes go all the way to the floor before i get any brakes. Sometimes it will do it and sometimes it won't...

Any one very last thing. I was sitting there at an idle and I was pumping the brakes to see if i could get the pedal to go the floor when I noticed that as I was pumping the brakes it engine's idle would slow way down...so the idle of the engine was being affected by the pumping of the brakes. This has to be something with the brake boost and the vacuum....

Any help would be appreciated...like I said I haven't had the chance yet to really investigate the problem...I just wanted some ideas of what it might be

Thanks
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Old 11-13-2006, 05:09 PM   #2 (permalink)
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I think you've cornered your problem to two of the three possible causes (assuming that you are not losing brake fluid). If you notice any reduction in fluid level in the resivior you should find the leak first.

Now assuming there is no EXTERNAL leaking you could have one of 3 problems 1: master cylinder, 2: booster or 3:LSPV (load sensing proportioning valve located under the bed on the passenger side, it has three brake lines running to it and a linkage connecting it to the rearend).

To test the booster you can pump the pedal with the truck off and it should not get closer and closer to the floor with each pump. Secondly, press and hold the pedal and then start the truck. If the pedal sinks towards the floor slightly the booster is probably working ok. Thirdly, run the engine for a few minutes to build good vacuum. Shut off the engine and press the pedal really slowly and firmly several times. The pedal should not go down as far the second and third times. Fourthly, press and hold the pedal with the engine running and shut off the engine while holding. If the pedal doesn't sink for 30 or so seconds then the booster is probably air tight. If all of these checks are ok then the booster is proabably not your cause. (this is all straight from the FSM).

I think the more likely cause is internal bleeding past an o-ring in either the master cylinder or LSPV. I personally had problems with both parrts that cause the sort of issues you are describing. I would first flush/bleed the entire system to ensure there is some good air free brake fluid in the system. If you still have the problem, start by rebuild/replaceing the master cylinder. I'm assuming that your's is original equipment as is mine on my 85. You can get them for 100 or so and it is cheap insurance.

The less likely issue but still possible culprit is the LSPV. They are adjustbile to change the brake bias towards the rear wheels. Mine was sticking at times such that rears would lock up to easily. I tried to replace mine with a junk-yard item off of an 87 4runner (they are pricey as the only source is Toyota). I was able to bleed the system but the valve caused my pedal to just go right to the floor with no braking ability. I tore my original apart (NOT recommended by toyota) and cleaned all of the crud out of it and put it back together. It works much better now as i'm still using the original.

Hope this sheds some light on things. I know there may be some cost involved but buying brake components is relatively cheap insurance to keep from plowing through somone's trunk.

-good luck
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Old 11-14-2006, 05:00 PM   #3 (permalink)
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i had a similiar problem in my 86 pickup. When i started the engine up the brakes were fine, but the more i drove it the less brakes i had. i limped it back home, bought a new master cylinder, and bleed the brakes like crazy.After all that they worked fine
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Old 11-15-2006, 12:47 PM   #4 (permalink)
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Quote:
Originally Posted by bilgedasto
i had a similiar problem in my 86 pickup. When i started the engine up the brakes were fine, but the more i drove it the less brakes i had. i limped it back home, bought a new master cylinder, and bleed the brakes like crazy.After all that they worked fine
Well I have plenty of brakes...I mean like the stopping power. It hasn't changed. But it is just a what point on the pedel I get the brakes...sometimes it is normal and other times it goes all the way to the floor before I get any brakes.

I will check it out next weekend. Thanks for all of the ideas
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Old 11-18-2006, 04:39 PM   #5 (permalink)
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I had very similar on another vehicle. It was in my case the master cylinder, it had a seal that was starting to fail. When it did not seal, fluid would get past it until I got to the bottom of the stroke where the cylinder wall was smoother and would seal (finally). Choose good brake fluid, it will be in there keeping rust out for a while.

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Old 11-18-2006, 05:10 PM   #6 (permalink)
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Quote:
Originally Posted by D'mon
I had very similar on another vehicle. It was in my case the master cylinder, it had a seal that was starting to fail. When it did not seal, fluid would get past it until I got to the bottom of the stroke where the cylinder wall was smoother and would seal (finally). Choose good brake fluid, it will be in there keeping rust out for a while.

D'mon
Sounds exactly like what is happening to me. Makes perfect sense. I am going to replace the master cylinder soon so I'll see what happens.


I was going to ask about something else...just out of curiousity. From what I have read, there is a one way vacuum valve between the brake booster and the manifold....correct? Now, what would happen if that were to fail or starting leaking other way? Would this cause the engine to run differently? I was just wondering because like I described before, when I was sitting at an idle and pumping the brakes the engine idle would slow down and run differently. I thought maybe if that valve was failing it would do something with the vacuum and cause the engine to run differently...
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Old 11-18-2006, 07:02 PM   #7 (permalink)
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The one way check valve allows air to flow from the booster to the intake manifold, but not the opposite. It's really there to maintain a vacuum for the booster incase the motor cuts out, allowing about 1-3 brake pedal "pumps" depending on the design of the system.

If the valve malfunctions and allows air to flow in both directions, it won't cause a vacuum leak or make the motor run any different, but you'll have no "assist" in the brakes if the motor is off since there will be no vacuum present in the booster. The dip in idle that you describe is actually normal. Engaging the brakes causes the brake booster to use up engine vacuum, thus creating a minor vacuum leak. If your motor was running too rich, it would have caused your idle to increase instead of decrease.

The main situation that causes a vacuum leak in the braking system is if the internal rubber diaphragm of the brake booster unit springs a leak. Sometimes it's old age, but in every case I've seen, it was a master cylinder that had gone bad, leaking brake fluid into the booster. The brake fluid eats away at the rubber diaphragm, and whala, a vacuum leak is created. This is exactly what happened on my 1959 Pontiac.
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