Brake pedal falls to floor during full lock left/right with steering wheel - Page 2 - Ford Powerstroke Diesel Forum
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post #11 of 27 Old 11-12-2019, 09:09 AM Thread Starter
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Got around to running 5 quarts of Mercon V through the system with no change. Guess next step is replacing the Master Cylinder or Hydroboost.
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post #12 of 27 Old 11-12-2019, 09:11 AM Thread Starter
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Quote:
Originally Posted by Hydro View Post
Problem is in the hydro-boost unit -- when you steer to lock it is applying too much pedal boost

Surprised you have not blown one of the rubber brake lines -- prolly is ballooning one of them
Why would I blow a rubber brake hose? The hydroboost is a separate system than the brake system. How would it cause too much pressure in the brake hose?
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post #13 of 27 Old 11-13-2019, 07:25 AM
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Quote:
Originally Posted by L-ight View Post
Why would I blow a rubber brake hose? The hydroboost is a separate system than the brake system. How would it cause too much pressure in the brake hose?
You stated that the pedal goes to the floor when the steering is turned to lock

The hydro-boost "helps" push the pedal down -- there is a mechanical connection between the pedal and MC, so even with the engine off, you can still push the master cylinder

The hydro-boost is doing just that -- pushing the master cylinder -- it should not assist more at steering lock tho
Question: if you lock/unlock the steering position -- does the pedal go up and down?

Now -- unless the brakes leak off -- all that extra pressure/volume has to go somewhere
Would be the same if you put both feet on the pedal and pushed real hard, enough to force the pedal to the floor

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post #14 of 27 Old 11-13-2019, 08:13 AM Thread Starter
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Quote:
Originally Posted by Hydro View Post
You stated that the pedal goes to the floor when the steering is turned to lock

The hydro-boost "helps" push the pedal down -- there is a mechanical connection between the pedal and MC, so even with the engine off, you can still push the master cylinder

The hydro-boost is doing just that -- pushing the master cylinder -- it should not assist more at steering lock tho
Question: if you lock/unlock the steering position -- does the pedal go up and down?

Now -- unless the brakes leak off -- all that extra pressure/volume has to go somewhere
Would be the same if you put both feet on the pedal and pushed real hard, enough to force the pedal to the floor
When I get out of the lock position the pedal stays soft until I pump it once. After that it is back to normal. If I press and hold the brake pedal down and turn to full lock the pedal stays. Once I let foot off pedal and reapply pressure the pedal falls to the floor. The pedal does not go up or down when I lock/unlock the wheel. Pedal stays the same until I let off. I have also tried pushing the pedal really hard with my foot and it stays pretty darn firm. I don't think I was able to get it to fall to the floor. You can def hear the fluid being pushed through the system when pressing hard on the brake pedal.
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post #15 of 27 Old 11-13-2019, 10:06 AM
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I found the following in an article about understanding hydroboost brake assist:

Quote:
2. Basic Function Test
To check if a hydro-boost system is functioning, perform this test:

With the engine off, pump the brake pedal until it is hard.
Start the engine.
Proper operation should result in the brake pedal sinking down and then pushing back up against your foot.
The sinking of the pedal when the engine was started is a result of the power chamber being pressurized. Once the power steering system is at full pressure, the pedal will push back against your foot pressure.

This test will only verify if the pump, hydro-boost unit and reserve are working. This test will not diagnose performance problems of certain customer complaints.
Conduct the test above. If your truck passes the above, I think that what is happening with your system is that at full lock the system pressure is falling off enough to affect the hydro boost. I don't know what could have occurred to cause this drop, but my guess is some crud got in a valve or orifice. I don't know squat about this system, but my hunch is that the power steering pump is at fault.

I've had trouble with turning the wheels with the engine off. You have to turn the steering wheel to turn the wheels. I didn't have any trouble with the brakes or steering, other than the mess.

Last edited by DaveP.; 11-13-2019 at 10:13 AM.
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post #16 of 27 Old 11-13-2019, 10:11 AM Thread Starter
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Quote:
Originally Posted by DaveP. View Post
I found the following in an article about understanding hydroboost brake assist:



Conduct the test above. If your truck passes the above, I think that what is happening with your system is that at full lock the system pressure is falling off enough to affect the hydro boost. I don't know what could have occurred to cause this drop, but my guess is some crud got in a valve or orifice.

I've had trouble with turning the wheels with the engine off. You have to turn the steering wheel to turn the wheels. I didn't have any trouble with the brakes or steering, other than the mess.
I have done this test twice with the exact same results which means the hydroboost works as it should. I mean I ran in total 7.5 quarts of Mercon V through the system with no improvement either. You're talking about stuff getting inside the hydroboost, yea?
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post #17 of 27 Old 11-13-2019, 04:10 PM
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Still trying to figure what turning the wheels to lock has to do with anything -- the hydro-boost unit only "helps" you push the brake pedal -- just like the vacuum booster does on a gasser -- there is a direct mechanical connection to the master cylinder

Only thing I can think of is the hydro-boost is "helping" too much -- but you differ in that thought
one other thing, would be if the booster is holding the brakes on, and the pedal is returning by spring -- will the truck move with the steering at lock and the pedal soft?

Don't confuse this system with the remote located hydro-boost system used on mid size trucks -- whole different kind of duck

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post #18 of 27 Old 11-14-2019, 04:02 AM Thread Starter
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Quote:
Originally Posted by Hydro View Post
Still trying to figure what turning the wheels to lock has to do with anything -- the hydro-boost unit only "helps" you push the brake pedal -- just like the vacuum booster does on a gasser -- there is a direct mechanical connection to the master cylinder

Only thing I can think of is the hydro-boost is "helping" too much -- but you differ in that thought
one other thing, would be if the booster is holding the brakes on, and the pedal is returning by spring -- will the truck move with the steering at lock and the pedal soft?

Don't confuse this system with the remote located hydro-boost system used on mid size trucks -- whole different kind of duck
When the pedal falls to the floor after lock it still brakes the truck, but just at the very end of travel. Obviously it does not brake as well as when the steering wheel is out of lock position. It's just when getting into that locked position the travel to brake is much greater and mainly at the end of the full travel the brake pedal is allowed to go before bottoming out. Oh I am not confusing them at all. I know a hydroboost is pretty much the same as a brake booster. The hydro just uses fluid instead of air.
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post #19 of 27 Old 11-14-2019, 06:09 AM
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I agree, hydro boost unit failing.

The power steering pump has to supply pressure to both the steering gearbox (not rack) and power brake hydro boost unit, and if the valve in the hydro boost unit that prioritizes the fluid pressure isn't working correctly, you get funny break and steering issues.

Our excursion needs the hydroboost replaced, it wants to pull to one side or the other randomly when the brakes are applied. Another symptom is hard steering when stopped, brakes applied, IE: parking lot maneuvers.

Edit: Reply posted before reading the 2nd page of posts, which include more information and further questions/answers. Sorry.

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Last edited by dieselcrawler; 11-14-2019 at 06:20 AM.
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post #20 of 27 Old 11-14-2019, 06:36 AM Thread Starter
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Quote:
Originally Posted by dieselcrawler View Post
I agree, hydro boost unit failing.

The power steering pump has to supply pressure to both the steering gearbox (not rack) and power brake hydro boost unit, and if the valve in the hydro boost unit that prioritizes the fluid pressure isn't working correctly, you get funny break and steering issues.

Our excursion needs the hydroboost replaced, it wants to pull to one side or the other randomly when the brakes are applied. Another symptom is hard steering when stopped, brakes applied, IE: parking lot maneuvers.

Edit: Reply posted before reading the 2nd page of posts, which include more information and further questions/answers. Sorry.

I already purchased the master cylinder, but I really don't think it is the problem. I will probably return this and grab the hydro instead. Is it bad I'd much rather change out the hydroboost over the master cylinder? Not a fan at all bleeding brakes/bench bleeding MC. Hydroboost you don't even have to remove the brake lines on the MC.
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