Brake issues on a diesel - bad brake booster or vacuum pump?

[MagooInFL]

I recently acquired a 1986 F350 with a 6.9 diesel.  Ive bled the brakes and I now get a firm peddle, BUT I am not getting brake assist.  The brakes are really tough to push down.  I pulled the main tube right off the vacuum pump and it has vacuum (I just put my finger over it, and could feel the suction) - but I understand that it may NOT be enough.  How do I know if the problem is brake booster or the vacuum pump?

[Gary Lewis]

Welcome!  

I don't know how much vacuum the diesel pumps are supposed to give, but I would assume that it would give something similar to the minimum vacuum a gas engine would show at idle, and that would be around 15".

Do you have a vacuum gauge?  If not, they are inexpensive and you can get one at most parts stores.  Then you may be able to put the vacuum gauge's hose inside the hose coming from the vacuum pump and test the output of the pump.

I will check the factory shop manual tomorrow to see if they have spec's on the vacuum you should expect.

[85lebaront2]

Gary, looking quickly at Alldata, it looks like it should pull 20" with no leaks. Does the low vacuum light illuminate? First thing suggested it check system integrity, it works brakes, cruise, HVAC and if a C6 the transmission vacuum modulator (I think). Some used a mechanical system from the injection pump.

[MagooInFL]

No AC on the vehicle but the heater works
No cruise control on the vehicle
It seems to shift fine and yes it is the C6

I have a harbor freight vacuum pump for bleeding brakes - could I reverse this onto the pump to get a vacuum reading?

[Gary Lewis]

Yes, you should be able to connect it to the pump and just read the gauge.

But the base heater doesn't use vacuum, so that doesn't prove you have adequate vacuum.  And if your C6 has a mechanical system that doesn't prove it either.  You should be able to see a vacuum line going to the tranny if it uses vacuum.

Bill - Good info.  Thanks.

[Ford F834]

Welcome to the forum 🙂

The most common issues with diesel vacuum supply are: pump diaphragm failure and the coffee can vacuum reservoir on the driver side inner fender likes to rust out.

As a side note, if your warning light is not on... and you indeed have no vacuum... get that warning light fixed! On mine the diaphragm failed and I had no idea until I exited the freeway and went to use the brakes. The only thing that saved the guy in front of me at the stoplight was the fact that I was towing my trailer with electric brakes. Fix the warning light and/or install a vacuum gauge in the cab 👍

[grumpin]

Ford F834 wrote

Welcome to the forum 🙂

The most common issues with diesel vacuum supply are: pump diaphragm failure and the coffee can vacuum reservoir on the driver side inner fender likes to rust out.

As a side note, if your warning light is not on... and you indeed have no vacuum... get that warning light fixed! On mine the diaphragm failed and I had no idea until I exited the freeway and went to use the brakes. The only thing that saved the guy in front of me at the stoplight was the fact that I was towing my trailer with electric brakes. Fix the warning light and/or install a vacuum gauge in the cab 👍

 Good info.. My 1988 F250 7.3, I used to own had the vacuum pump seize and throw the belt. Don't remember a warning light after it happened.

[Ford F834]

grumpin wrote

Good info.. My 1988 F250 7.3, I used to own had the vacuum pump seize and throw the belt. Don't remember a warning light after it happened.

The really bad part of this is that when the warning system fails, it seems to fail in a way that indicates everything is good when it’s not. If I were going to continue to run vacuum brakes on my diesel I would install a mechanical vacuum gauge as a back up for the warning light. In my case, I am deleting the problem by switching to hydroboost. If the pump fails, loosing my HVAC controls and cruise control is not that much of an emergency.

[MagooInFL]

Yeah - there is no warning light on.

I like the idea of installing the dash gauge!  Never thought of that - don't know why.  I had the vac pump go out on my '85 F250 - never saw a warning light then either - I had a mechanic diagnose that issue for me.  But I am trying to do more and more on my own.

[MagooInFL]

I used my harbor freight vacuum pump for bleeding brakes and hooked it up to where the vacuum tube connects directly to the brake booster - I had 25 lbs there.

As a secondary test - I pumped the brakes a couple times - then started the engine and the peddle didn't move at all.  Is that the definitive test in my case?  bad brake booster?

[Gary Lewis]

I'm not sure I understand.  Did the vacuum pump on the engine pull the 25", or was that using the HF pump?  I think you used the truck-mounted pump, but don't want to assume anything.

And on the secondary test, you pumped the brake with no vacuum on the system and then started the engine and the brake pedal didn't move?  I wouldn't expect the pedal to move by itself just because vacuum is applied.  So I don't think that proved anything.  Or, did I misunderstand?

[Gary Lewis]

The General Hydraulic Brake System chapter of the 1985 factory shop manual says:

1. With the engine stopped, eliminate all vacuum from the system by pumping the brake pedal several times.  Then push the pedal down as far as it will go, and note the effort required to hold it in this position.
 If the pedal gradually moves downward, the master cylinder is leaking internally and should be re-built or replaced.

2. With the pedal pushed all the way down, start the engine.  If the vacuum system is operating properly, the pedal will move downward.  If the pedal position does not change, the vacuum system is not operating properly and should be subjected to a vacuum system test.

That means we go to the Brake Booster Vacuum Pump - 6.9L Diesel Engine chapter.  That has two tests, the first of which says that the pump should pull 21 inches, minimum, at sea level within 30 seconds of starting the engine and running it at idle - with just the gauge hooked to the pump.  (But, there is a reduction due to altitude chart, which shouldn't apply to FL.)  So that looks good.

The next step says that you reconnect the vacuum system but put the gauge in via a tee.  Start the engine and you should have vacuum within 3" of the previous reading.  But if the vacuum is more than 3" low you should replace the brake booster.

Does that make sense?

[MagooInFL]

Yes - that all made sense ( I think)
Just to clarify what I previously tested - I hooked the vacuum pump up to the hose that runs to the brake booster and then started the engine - then I read the gauge at 25lbs - so I didn't hook it up and then pump the vacuum by hand

so I need to do the same test but put a "T" fitting in between the vehicle vacuum pump and the brake booster and then get a reading

[Gary Lewis]

Ok, you did the first step of testing the pump.  Now you need to connect the pump to the brake booster and have the gauge teed into that line.  I'm guessing that you'll have more than a 3" drop in vacuum.

But don't forget to also dump all the vacuum and push the pedal all the way down.  Then start the engine and see if the pedal drops like it should.  If the booster isn't functioning properly the pedal won't drop.

[MagooInFL]

Ok - simple question - how do I “dump all the vacuum”?

On Oct 8, 2018, at 3:45 PM, Gary Lewis [via Bullnose Enthusiasts] <[hidden email]> wrote:

Ok, you did the first step of testing the pump.  Now you need to connect the pump to the brake booster and have the gauge teed into that line.  I'm guessing that you'll have more than a 3" drop in vacuum.

But don't forget to also dump all the vacuum and push the pedal all the way down.  Then start the engine and see if the pedal drops like it should.  If the booster isn't functioning properly the pedal won't drop.

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[Gary Lewis]

That's what happens in Step 1. "With the engine stopped, eliminate all vacuum from the system by pumping the brake pedal several times."

[MagooInFL]

OK - NOW someone should be able to tell me the answer to the question that is looming.
I installed the vacuum gauge - and after dumping the vacuum and holding my foot on the brake peddle and then starting it up the peddle doesn't drop at all.  BUT the vacuum does climb even with my foot on the brake, then when I release it climbs to 25 then when I press the brakes - it drops from 25 down to zero.

what does that mean?  bad vacuum pump or bad brake booster?

Thank you so much for all who have contributed!

[MagooInFL]

OOPS - let me change that - when I dump the vacuum and then start it it HOLDS at ZERO, then when I let off the brakes it slowly climbs to 25 then when I push the brakes it drops down and the needle bounces between zero and 3 until I let off again!

[Ford F834]

Sounds like the booster diaphragm is bad...

[Gary Lewis]

Yes.  The diaphragm is killing the vacuum.  The maximum drop allowable is 3", and it took it to 0.

And, it failed the "pedal should drop when the engine starts" test.

Now you know what to fix w/o throwing parts at it.