what do i need to know about hydro boost brakes?

[77bronco]

They went to hydroboost on the diesels to get away from having a separate vacuum pump to assist the brakes. When I had a '87 6.9 diesel Ford it had a vacuum pump running off the crank pulley to provide the vacuum needed for the brakes and HVAC controls. It got pretty hard to stop when that pump went bad...
Not sure if the gas trucks have hydroboost in the F-450, but my F-350 V-10 has standard power brakes.

 

[Highbeam]

Despite using hydroboost, the Ford super duties still have a vacuum pump to run the HVAC and "auto" 4x4 hubs. The vaccum hubs are a dismal failure but do have a manual locking feature. The new vacuum pumps are little electrical deals on the pside fender.

Brake system is brake fluid, power steering/hydroboost fluid is atf. Transfer case is atf too.

 

[BrentB]

I found this old thread about hydroboost brakes, so thought I would tag onto it. I have a 2003 Chevy K1500 HD quadrasteer with hydroboost brakes. the first 50% of pedal travel seems normal, then i hit what feels like a detent, and can then push through all the way to the floor. I hear a slight hissing beyond 50% travel. I replaced the master cylinder but no difference.
1. Can anyone else with this vintage Chevy confirm that the brake pedal should be firm when brakes are applied?
2. Do you think my booster is leaking?

thanks,
Brent

 

[Stimw]

I've owned several P-30 (1 ton Chevy) motorhomes with hydraboost brakes and the peddle always felt like normal brakes.
I had a 1983 Winnebago that If I nailed the brakes on dry pavement it would skid ALL 6 wheels!! (Like when 5 SOB's pass then cut in front at the same time and slam on their brakes for a red light!)
Don't know what would make a "hissing" sound unless the belt is slipping. ???

 

[Soundguy]

power steering do anything funny dureing that time?

 

[BrentB]

Nope, the power steering feels and acts just fine. I did google and find a thread that said a hissing noise was normal. Since there is no vacuum lines there,it must be hyd fluid bypassing somewhere. It sounds a lot like when you turn your steering wheel all the way against the stop.

I found a booster test procedure where you pump the brakes with engine off, then hold with 40 lbs force. then start engine and pedal should drop slightly then come back up. mine doesn't do this.

The truck stops fine, and I rarely tow...but hate to risk something being wrong when I do want to tow something. Thanks for any thoughts!

 

[aczlan]

Nope, the power steering feels and acts just fine. I did google and find a thread that said a hissing noise was normal. Since there is no vacuum lines there,it must be hyd fluid bypassing somewhere. It sounds a lot like when you turn your steering wheel all the way against the stop.

That is what I have observed on 2004ish F350 and a 2005 Suburban with hydroboost.

Aaron Z

 

[Soundguy]

reason I asked about the PS is, as you know.. the ps provides the power for the boost since it is not vac boost.

here's a link that may or may not help any

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Nope, the power steering feels and acts just fine. I did google and find a thread that said a hissing noise was normal. Since there is no vacuum lines there,it must be hyd fluid bypassing somewhere. It sounds a lot like when you turn your steering wheel all the way against the stop.

I found a booster test procedure where you pump the brakes with engine off, then hold with 40 lbs force. then start engine and pedal should drop slightly then come back up. mine doesn't do this.

The truck stops fine, and I rarely tow...but hate to risk something being wrong when I do want to tow something. Thanks for any thoughts!

 

[Soundguy]

link title is odd.. but it clicks thru anyway.. just mouse over itto see where it goes .. etc.

 

[BrentB]

thanks for the link! sounds like the noise is normal..
the link said:
The hydro-boost will emit normal hissing noises when above-normal brake pedal efforts are applied (40 lbs. and up). The hiss sounds particularly noticeable with the vehicle motionless and will increase in intensity as pedal pressure increases above 40 lbs. and system operating temperature increases. Loud hissing sounds at or below normal (20 to 25 lbs.) pedal effort warrants investigation.

so the only thing that seems odd is that I can push the pedal to what feels like to the floor...with what I estimate to be much less than 100 lb effort (of course maybe I'm pushing harder than I think!) It does not feel like it is firming up with brakes applied. I bled brakes 3 times and pads are good, so it shouldn't be air in the lines, although I have no way to bleed the ABS valves.