US1 Fountain

iTrader: (1)

Working on sisters '03 GMC 3500 DRW w/ Duramax/Allison.

What/how exactly do the brakes work as far as power assist?
I've never dealt with this setup.
Appears the PS pump supplies the power assist for the brakes at the master cyl. by means of some booster contratraption behind the MC where normally a vacuum booster would mount. Is this a better system or do diesels not build vacuum like a gas motor?

I take it that there is no common fluid between the master cyl and the booster pump---- Brake fluid in master cyl, PS fluid in booster pump.
She stated her brakes have felt spongy like they have air in lines. The chevy dealer replaced both PS hoses to the booster pump saying they were leaking. I can see that causing the brakes to be harder to push if leaking, but not creating a spongy pedal. What's the scoop here?

88bullet

iTrader: (1)

its called hydro-boost. the p/s pump supplies the assist to your hydro boost unit which assists in power brakes. as for the spongy pedal they all feel like that when sitting still. the pedal comes back when moving. my 04 duramax is the same. unless she is getting unwanted abs activation when coming to a stop and usually turning less than 7mph. that means she needs either front wheel speed sensors or hubs if the bearings are bad

509 SC

I have an 06 and have always thought the brakes had a weird feel when activated, almost like they're fighting back at you. It does stop fine but the pedal response is strange. The truck had new rotors installed from the dealer when I bought it 2 years ago, 15,000 miles since. Anyone else have a similar experience?
Steve

US1 Fountain

iTrader: (1)

So there is no direct mechanical linkage between the brake pedal and the actual master cyl itself, just passing thru the booster? Or is the booster the only direct link to the master cyl itself? The latter would explain a spongy pedal.

I'm thinking since this is her vehicle just used to pull her horse trailer once/month, that going from the daily driver Explorerer to the GMC gives her the false feeling of brake problems?

Thanks


Why is it that 90% of the rigs on road towing horse trailers, are driven by women?

88bullet

iTrader: (1)

Yes there is a rod from the pedal to the master but the power steering pressurized fluid is what gives you the assist done with hydraulic pressure and valving in the hydroboost unit

Rippem

brakes are my only real complaint with my 06

rear calipers siezing up, e-brake, warping junk rotors. granted I don't drive it much but WTF? I've had a lot of low driven mile vehicles and this one is just a PITA- does not like to sit!

now I have a bad front sensor too

there is a great thread on dieselplace about how to service and conquer alot of the issues with that gen truck's brakes

silicone brake grease on the slider pins regularly is a must and I have been slacking

aquaforce

GM uses HydroBoost on 3/4 ton stuff gas or diesel especially when the chassis rating is high. This is a great bridge technology between vaccum boost hydraulic brakes and big truck air brakes.

I have had it on my last three trucks and have worked on this stuff throughout my career. Some drivers have to get used to it and some never do and tear it up. For example, the ones who tear it up stop short and exceed the system limits with no trailer brakes on a unit that is towing at full limited rating. They have a lot of Hydro system failures and blame it on the truck. The pedal feel is not at all an indicator of brake performance but most drivers are used to typical hydraulic brakes where that is the case. If hydroboost is driven like this then failures are common and braking performance is destroyed very fast by glazed friction material requiring higher brake pressures.
In a fully functional system where there are no problems, the spongy feel is the hydraulic throttling of the assist pressure to keep the apply pressure low. In a system where there are braking performance problems or a needed repair then this is an indicator of other things.


Ok, I suppose this ist TMI and not what one is looking for but I said all that to say this, look under the carpet where the brake pedal rod runs through the bulk head. If there is any fluid leaking there then the hydroboost unit itself has internal leaks, from crushing the pedal to hard and too often, and will have to be replaced.

Good luck with all this.



On my 06 LBZ truck the PO towed a flat bead goose neck with no trailer brakes and I have had to fix all the issues resulting from this abusive use of the brakes. This dually could stop it fine but the hydroboost pedal pressure had to be crushed to do it. To a typical driver of vaccum hydraulic brakes it would feel just fine but with hydroboost that is crushing the pedal and running all the pressures at max boost all the time.

Rippem

there was a bad run of leaky boosters put in these truck too forget what year 05/06

Unlimited jd

iTrader: (5)

low on power steering fluid or air in the system will cause the pedal to kick like that pretty comman issue

Unlimited jd

iTrader: (5)

if you drive a gm daily and get in a ford most likely your first stop is gonna throw you to the windshield lol, not neccesarily a superior braking system just a much firmer pedal feel