Brake Pedal drops to floor when truck starts.. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Jefe · 08-25-2003, 01:37 AM

(Drum brakes all the way around) This thing has me stumped...need some serious help. The pedal can be pumped up when the motor is not running and stays really stiff, doesnt fade, not mushy, and doesn't drop. When truck is started it drops almost to the floor. New parts less than a week old include, power brake booster,new power brake booster check valve, master cylinder, front two wheel cylinders, new shoes and drums on all four wheels, new front rubber brake lines. I'm inclined to think it is a vacume booster problem because it only goes down when motor started. It is connected to the back of the carb. and is drawing about 13-15 inches of vacum at idle. Any ideas are completely welcome!! Thanks guys!!

Douglas.

JimKshortstep4x4 · 08-25-2003, 06:33 AM

Do you mean that the pedal goes to the floor when you start the truck by itself? Or do you have to step on it and then it sinks down?

Jim

70c10 · 08-25-2003, 01:02 PM

I have heard of remans being bad out of the box.

neonlarry · 08-25-2003, 02:53 PM

What Jim said and do you get any pedal after it has run for awhile? Also you mentioned you attached to back of carb, ported vacuum or full manifold vac?

orange72 · 08-25-2003, 02:59 PM

i had a booster one time that would sink to the floor when you steped on it. it was weird cause it would stop the truck by itself. we got a new one though so i dont know why that new one would be messing up.

Jefe · 08-25-2003, 03:14 PM

When I step on it, it goes straight to the floor, still stops the truck but slowly. All the brakes are adjusted out to the drums properly, and the lines are all bled to where there is no air in them. When I turn off the truck, I pump the brakes a little and the pedal comes right back, brakes feel great.

Thanks!

Douglas.

chevyhouse · 08-25-2003, 03:17 PM

i had this problem when I did power on my truck, went through a new booster and vacuum checking and all sorts of stuff with the tech rep. turns out that I had installed the booster arm on the wrong side of the pedal, making a very slight angle into the booster. I clearly remember putting it on the wrong side when I installed it because it was so much easier to access. It took me a while to remember where I had cut the corner but as soon as I thought of it I changed it and it fixed everything. good luck

lukecp · 08-25-2003, 05:55 PM

I would guess the master cylinder.....even if it is new. Did you bench bleed the master cylinder before you put it in?

I have bled my truck with PB before, and the pedal will feel stiff with air in the lines, but upon engine startup, it gets excessively mushy.....My guess is that you still have air in the lines somewhere, or your new MC is bad.

JimKshortstep4x4 · 08-26-2003, 07:46 AM

I agree with Yellow Custom, there is air remaining in the brake system. Usually the rear is very difficult to bleed, but I would rebleed the brakes starting at the prop valve and doing the front and rear bleeders.

Jim

tom hand · 08-26-2003, 08:36 AM

It could be the freeplay...it the matercylinder is preloaded by the booster it is impossible to bleed. The pedal should move at least 1/2 inch before it does anything. Loosen the M/C bolts and if it pushes away from the booster you found the problem. I believe you can adjust the rod in the booster

08-25-2003, 01:37 AM	#1
Jefe Registered User Join Date: Jul 2000 Location: Chandler, Arizona, USA Posts: 299	Brake Pedal drops to floor when truck starts.. (Drum brakes all the way around) This thing has me stumped...need some serious help. The pedal can be pumped up when the motor is not running and stays really stiff, doesnt fade, not mushy, and doesn't drop. When truck is started it drops almost to the floor. New parts less than a week old include, power brake booster,new power brake booster check valve, master cylinder, front two wheel cylinders, new shoes and drums on all four wheels, new front rubber brake lines. I'm inclined to think it is a vacume booster problem because it only goes down when motor started. It is connected to the back of the carb. and is drawing about 13-15 inches of vacum at idle. Any ideas are completely welcome!! Thanks guys!! Douglas. __________________ -1968 Chevy SWB = Frame Off/383/Muncie Car 4 speed -1968 Chevy SWB = numbers matching 307/Truck 4 speed. -1966 Chevy II = 2 door No Post/130,000 Orig miles/230 6cyl/powerglide/Vintage AC -1972 Chevy Nova = 2 door/412 Small Block -1927 Ford Roadster = Homebuilt/widened 8"/350/ Custom Everything.

08-25-2003, 01:02 PM	#3
70c10 67-72 Addict! Join Date: Feb 2002 Location: The TAX State! Posts: 7,857	I have heard of remans being bad out of the box. __________________ Jim 1970 C/10 Fleetside w/Ghost Flames Lowered 4.5" front and 4" rear (Raked) 355/350 Turbo w/shift kit 10" Redneck Performance Verter w/2500 stall Hooker Super Comps part#2808-1 Performer RPM Air-Gap 12 Bolt w/3:73 gears- Eaton Posi Comp Cams XE262 with 1.6 Crane Energizers, Road Demon 625 and Brodix IK 180 heads

08-25-2003, 02:53 PM	#4
neonlarry Registered User Join Date: Feb 2002 Location: Vacaville, CA Posts: 2,746	What Jim said and do you get any pedal after it has run for awhile? Also you mentioned you attached to back of carb, ported vacuum or full manifold vac? __________________ 70 C/10 Light Red 350/TH350, HEI, Duals w/40 series Flows, 91 seat, LED taillights 99 Pontiac S/C GTP, SLP Ram Air hood, GMPP Konis & springs 95 Neon ACR, MP PCM, AFX UDP, 3.0 CAI

08-25-2003, 02:59 PM	#5
orange72 Registered User Join Date: Sep 2000 Location: huntington,texas,united states Posts: 399	i had a booster one time that would sink to the floor when you steped on it. it was weird cause it would stop the truck by itself. we got a new one though so i dont know why that new one would be messing up. __________________ 1972 chevy long bed. Currently under construction!! Bagged, 383/700r4.

08-25-2003, 03:14 PM	#6
Jefe Registered User Join Date: Jul 2000 Location: Chandler, Arizona, USA Posts: 299	When I step on it, it goes straight to the floor, still stops the truck but slowly. All the brakes are adjusted out to the drums properly, and the lines are all bled to where there is no air in them. When I turn off the truck, I pump the brakes a little and the pedal comes right back, brakes feel great. Thanks! Douglas. __________________ -1968 Chevy SWB = Frame Off/383/Muncie Car 4 speed -1968 Chevy SWB = numbers matching 307/Truck 4 speed. -1966 Chevy II = 2 door No Post/130,000 Orig miles/230 6cyl/powerglide/Vintage AC -1972 Chevy Nova = 2 door/412 Small Block -1927 Ford Roadster = Homebuilt/widened 8"/350/ Custom Everything.

08-25-2003, 06:33 AM	#2
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	Do you mean that the pedal goes to the floor when you start the truck by itself? Or do you have to step on it and then it sinks down? Jim

08-25-2003, 03:17 PM	#7
chevyhouse BEER-Vacation in a can... Join Date: Jun 2001 Location: wichita, ks Posts: 444	i had this problem when I did power on my truck, went through a new booster and vacuum checking and all sorts of stuff with the tech rep. turns out that I had installed the booster arm on the wrong side of the pedal, making a very slight angle into the booster. I clearly remember putting it on the wrong side when I installed it because it was so much easier to access. It took me a while to remember where I had cut the corner but as soon as I thought of it I changed it and it fixed everything. good luck __________________ Kurt 03' Trailblazer 69' C10 "SPSHL-K", 388 stroker/BTO 700R4, 2400 stall, 3.73 posi, lwb, fleetside long box, 3.5"/5" drop, 18" Budnik 2000 Yamaha V-Star 1100 Custom For Sale-87' V10-4x4, 408/700r4, 4.10's PM for pics

08-25-2003, 05:55 PM	#8
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	I would guess the master cylinder.....even if it is new. Did you bench bleed the master cylinder before you put it in? I have bled my truck with PB before, and the pedal will feel stiff with air in the lines, but upon engine startup, it gets excessively mushy.....My guess is that you still have air in the lines somewhere, or your new MC is bad. __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

08-26-2003, 07:46 AM	#9
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	I agree with Yellow Custom, there is air remaining in the brake system. Usually the rear is very difficult to bleed, but I would rebleed the brakes starting at the prop valve and doing the front and rear bleeders. Jim

08-26-2003, 08:36 AM	#10
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	It could be the freeplay...it the matercylinder is preloaded by the booster it is impossible to bleed. The pedal should move at least 1/2 inch before it does anything. Loosen the M/C bolts and if it pushes away from the booster you found the problem. I believe you can adjust the rod in the booster __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS



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