1996 GMC Suburban Grabbing brakes when parts are cold

AUTOBUM

MEMBER
1996 GMC SUBURBAN
V8
2WD
AUTOMATIC
140,000 MILES

Hi guys,

I have a brake issue. Upon initially stepping on the brakes when they are cold, they will grab badly for the first several applications. Once they have warmed up, they are less "Grabby", but still are not right. I have replaced the shoes and rotars about 15k miles ago with mid-quality parts. I don't recall changing out the calipers at that time. The problem has begun a while back. The vehicle does a lot of stop and go-city driving. And I have advised my wife to avoid riding the brakes to eliminate glazing, as to whether or not she followed my advice, I don't know. Most likley not.
Any advice would be greatly appreciated

Regards,
Autobum

3 Replies

MERLIN2021

MECHANIC
17,250 POSTS

Check the fluid level and condition, you may have contaminated fluid, put some fluid from the master in a cup of water, swirl, it should turn milky white, with no oil beads floating on top. If any beads, the fluid is contaminated with oil or tranny fluid! This would require the replacement of ALL rubber brake parts, calipers, Master, wheel cylinders and hoses, then you would need to flush the steel lines with alcohol or brakekleen. Does the pedal return normally? This probably has a hydraboost unit, powered by the power steering pump, and not vacuum. Any P/S troubles? Whinning pump? Another possibilty is grease on the shoes or pads, any axle/brake fluid leaks in the rear? Last brake job for front? Rear?

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Saturday, October 31st, 2009 AT 1:10 PM

AUTOBUM

MEMBER
3 POSTS

Thanks for the quick reply Merlin. I appreciate it very much.
Now that you mention it. Yes the PS pump has begun to whine loudly for about 2 weeks (new in comparison to the brake grbbing issue), but is not loosing fluid. The pump is relatively new. About 1.5 yrs old.

How would the brake system become contaminated with oil or tranny fluid? I am a bit behind the curve on later model technology and expected the two systems to be completley seperate. Does it cross over at the booster?
If this is the case, and the system is contaminated, then is my PS pump now worthless or can it be salvaged?

Thx, AutoBum

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Saturday, October 31st, 2009 AT 9:59 PM

MERLIN2021

MECHANIC
17,250 POSTS

First things first...test the brake fluid as described, , but the hydraboost may need to be replaced.
Condition/Concern: On vehicles with hydro-boost, customers may notice during full lock turns or near full lock turns that the brake pedal will move down or self apply. This can be caused by the internal dump valve of the hydro-boost not seating properly due to contamination in the steering system. The hydro-boost assembly is not tolerant to certain types of contaminates that may be present in the steering system. � Recommendation/Instructions: The Hydro-boost system is not serviceable, and must be replaced. Before replacement of the hydro-boost system, the entire steering system (power steering pump, lines, and the gear box) must be flushed to avoid reoccurrence. � Source: GM Preliminary Information Bulletin #PIT3665A - (Jul 1, 2008)
Testing:
PRELIMINARY CHECKS Check engine idle speed. Check all power steering hoses and brake lines for leaks or restrictions. Fill master cylinder with brake fluid. Fill power steering pump reservoir with power steering fluid. Check for air in power steering fluid reservoir. Check power steering belt tension and condition. Check power steering pump pressure. ACCUMULATOR LEAK-DOWN TEST 1. Start engine. Charge accumulator by applying brakes or turning steering wheel from stop-to- stop. Turn engine off. Allow vehicle to sit for one hour. Apply and release brake pedal. There should be at least 2 power-assisted applications. This indicates the accumulator is retaining a charge. 2. If accumulator does not retain charge for one hour but functions normally immediately after charging, replace accumulator valve. See OVERHAUL . If accumulator can be heard charging and discharging but does not hold a charge, replace accumulator valve. See OVERHAUL . 3. Discharge accumulator by pressing brake pedal several times. Attempt to rotate accumulator can in respect to housing. If it is possible to rotate can, accumulator has lost its gas charge. Replace accumulator assembly. BOOSTER FUNCTIONAL TEST With engine off, press brake pedal several times to deplete accumulator reserve. Press and hold brake pedal with about 40 lbs. (18 kg) pressure. Start engine. Brake pedal should fall slightly, then push back against foot. If no action is felt, booster is not operating properly. NOTE: Inoperative Hydro-Boost cannot cause noisy brakes, fading brake pedal or pulling brakes. If one of these conditions exists, other components of brake system are at fault.
11/1/2009 ...
Bleeding:
BLEEDING HYDRO-BOOST SYSTEM 1. Fill power steering pump reservoir and leave undisturbed for at least 2 minutes. Start engine and run momentarily. Add fluid if necessary. Repeat until fluid level remains constant with engine running. 2. Stop engine. Raise front wheels off the ground. Turn steering wheel from stop-to-stop. Add fluid if necessary. Lower vehicle. 3. Start engine. Press brake pedal several times while turning steering wheel from stop-to-stop. Turn engine off. Press brake pedal several times to exhaust accumulator pressure. Check fluid level. Add fluid (if necessary). 4. If fluid is foamy, let vehicle stand for several minutes, then repeat step 3). Air in system will cause fluid level to rise with engine off. Continue to bleed system until all air is expelled. If all air is not expelled after bleeding system, check power steering system for problem.