99 Yukon brake pedal becomes hard to press when braking suddenly. Power assist prob?

[MrSpicy]

Hello guys I've got what could be a bit of a serious issue with my 1999 GMC Yukon (especially if my wife is driving). Recently I've noticed that when stopping, mainly when suddenly braking or coming to a quick hard stop (because people cant drive and like to pull out in front of you and then go slow) the brake pedal becomes very hard to press, just how it feels when you step on the brakes with the car off.

My best guess is that something is going wrong with the power assist for the brakes. Also when braking normally this doesn't happen. The vacuum line looks to be ok but I will take it off and check tomorrow when the engine is not hot. Any other thoughts? I need to get this fixed super ASAP I start a new job Wednesday and will be driving 60+ miles per day. Thanks for any help or info.

 

[SunlitComet]

sorry pal, but if your vacuum source line is intact and not leaking then your booster internal valve is screwed up. booster replacement is required. at some point it will start leaking as well and effect the engine running when you press the brakes. like raising the idle as if you were pressing the gas at the same time. i would normally recommend to anyone who has the time convert to hydro boost but i your case get a new vacuum booster and replace it. it is going to give your hand some cramps getting it done. but you will be back to normal with a sound piece of mind. wife too.

 

[MarkD51]

As Sunlit Comet has suggested, check the vacuum hose, and connections from engine to Power Booster. Also check the Check Valve and Grommet on the Booster Face as well. While rare, a bad Check Valve and Grommet can also cause loss of proper power assist.

If you determine you need a booster, I just did this myself a couple months back, and Sunlit helped walk me through with tips.

Two tips I can pass on, it was certainly easier completely removing the Master Cylinder prior to removing the Booster. One thing I did, was once disconnecting the two brake lines from the MC, I quickly capped them off with two plugs, and these can be gotten with a MC Bleeder Kit from any of the big auto parts stores for not much cash. At most, I lost maybe 5-6 drops of brake fluid total. Seemed fine after, and although I did re-bleed the brakes, they seemed fine before the bleed.

The other, get the brake switch clip kit too, or go to GM dealer and buy the proper clip. They're usually damaged, or lost during removal, best to have a fresh new clip to pop back on the Pedal Stud once re-installing the switch back onto the Pedal.
Hope this helps.
Mark

 

[MrSpicy]

Alright guys thanks for the advice, looks like worst case scenario. I'm going to try replacing the vac line and the valve too just for the slight chance that it fixes it, and if it doesn't at least I would have probably replaced it when I changed out the booster anyway... it is a 15 yr old hose. Any threads on here that would be helpful? Also What's the difficultly level and how long should I expect it to take? Thanks again guys.

Oh also any recommendations on which one to buy?

 

[SunlitComet]

hardest part is getting the pushrod and brake switch off as well as accessing the four 15mm nuts on the inside if firewall that hold the booster on. various socket extensions , deep socket & universal joints will go a long way with some minor patience. standard labor book time is 2.5 hours.

 

[MrSpicy]

Thanks again for the advice. My wife borrowed her moms car and ive been taking her Camry to work so it will wait till this weekend or maybe next if I need to order anything. This hydro-boost upgrade does sound tempting but it sounds like it would take more time then I probably want to spend on it. How much modification does it take? sounds like a new ps pump is needed?

Also can anyone recommend what booster would be good to purchase if I don't go through with an upgrade? Worth replacing the master cylinder while im at it?

 

[MarkD51]

I believe I went with a Cardone Power Booster from AZone. Cost was something like $113.

As for a new Master Cylinder, Sunlit passed on some tips to me about such, to be careful what you buy. I think he said to stay away from rebuilt, buy new, and preferably something like AC Delco, or another, forget which but to stay away from certain "house brands".

Hopefully you two can share a PM on such choices.

My MC looked like new, no leaks, truck is low mileage,thus I didn't change mine.
For sure if changing the MC too, you'll need the little bench bleeding kit, about $10, it's not hard to do, best to set in a Vice while you bleed.
There's tons of youtube vids showing both a booster swap as well as bench bleeding an MC. The Silverado Truck-Suburban-Yukon from the same era will be a virtual identical set up, and procedure also.

 

[SunlitComet]

a1 cardone have done good in the spare parts are. of course a gm subsidiary brand would be best like delco or delphi for example.

for hydro you need a pump from a diesel or big blocked vehicle. besides the booster you will need the specific bake peddle for it. switch is same. you will replace some and add some new lines for the system. you will also want to acquire the oil cooler for it. preferably the one that is a finned two row the mount to the left of rad behind the grill. the matching mc to go with the booster. some might say a jd5 or jd6 booster is good enough but personally i use a jb8 booster. my parts list is based on a 3/4 ton burb with big block7.4 l in it. being as you have the evo steering system a change may be needed to disable it or some module change to keep it working. if you keep it valve on the pump then that line is a different routing then a pump without it.

hope that helps.

 

[MrSpicy]

Thanks and thanks again guys definitely helps, I think ill have to talk my self into passing on the Hydro upgrade, if this was a month ago I would do it but with my new job the only free time I have is on the weekends now and in order to keep my wife as well as my Yukon happy. So for the sake of time Ill just keep it stock. Ill look around and see what's good. Thanks again.

 

[MrSpicy]

ok I think im going to go with a cardone remanufactured booster $89 on amazon and a new cardone master cylinder $35. There is an AC delco (made by durrastop) for $54 but I don't think it will make much difference. Any thoughts guys? Also for the brake switch clip is it just one of those c looking clips or is it something else? Thanks.