Truck creeps forward at red lights

[jimbo006]

I am having problems with my 91 sy. The truck stops fine but when I am sitting at a light the truck will start to creep forward. I am keeping steady pressure on the brake pedal when it starts to creep again so I push a little harder and it will stop it for a couple of seconds and then it will start to creep forward again...any ideas??? I put a new master cylinder on the truck, the brake pads and shoes look fine as wel as the rotors and drums.

While I am here, I am also having a problem with my transmission hitting my floorboard when I have a passenger in the truck. When I am running around by myself it is fine but when someone rides with me you can hear and feel the trans hitting something...it is frustrating and embarrassing. Any ideas about that would be great too.

 

[dgoodhue]

Re: Truck creeps forward at red lights

If the brakes are bled properly, it could be the brake booster leaking. Do you hear a hiss when sitting still.

As for the tranmission floor board noise, I would check the torsion bar mounts first. The commonly go and make noise such as you describe.

 

[SacDoug]

Re: Truck creeps forward at red lights

Your truck creeping forward could be due to air in your brake lines or a vacuum leak at your brake booster. Check the vacuum hose going to your brake booster for cracks and hardness. If it is either replace it. If it is good, then bleed all air out of your brake system.

Your tranny hitting the floor board sounds very bad. Are you sure it is the tranny? If so, your motor mounts are gone.

 

[TurboJunk]

Re: Truck creeps forward at red lights

what does it idle at ? maybe the idle for some reason is too high and your not use to pressing down on the brakes that hard

 

[jbone]

Re: Truck creeps forward at red lights

The tranny problem is probably what dgoodhue said. I have the same problem and its the torsion bar mounts I just havent gotten around to fixing them.

 

[jimbo006]

Re: Truck creeps forward at red lights

As far as I know there is no air in the brake system and no leaks on the brake booster, but I do sometimes hear a little whistling sound when I get off of the gas and also sometimes when I am stepping on the brake. The truck idles at around 700 rpm according to the dash tach which I am not 100% sure if it is accurate. I plan on looking at the torsion bars this weekend. Wish me luck...

 

[Black Knight]

Re: Truck creeps forward at red lights

The whisteling sound might be your brake booster check valve, pretty common to go bad. From what you are saying if the pedal is high then I really doubt you have air in the lines or when you depress the pedal it would compress the air and go lower than normal. One possible thing could be master cylinder not holding pressure, i'll try to get more info on this later and poss other ideas. HTH

 

[Daron]

Re: Truck creeps forward at red lights

I had the creep... Master Cylinder replacement fixed it.

 

[bezerk]

Re: Truck creeps forward at red lights

or get a higher stall TC

 

[jimbo006]

Re: Truck creeps forward at red lights

Does anybody know the part number of the brake booster check valve and where to get it.

 

[Black Knight]

Re: Truck creeps forward at red lights

GM #18022219 its probably $20-$25 from the dealer or you can buy it from a parts store for probably half that, but rumor has it they don't look the same, if that matters to you.
Are you sure that is the problem though before we start throwing parts at it?? The best way people describe a bad check valve is it's sounds like a duck quaking or honking, yah its kind of ridiculous but thats all I got.

 

[Black Knight]

Re: Truck creeps forward at red lights

If the pedal is staying high you probably your pulling forward it might be a frozen caliper, if pedal drops a little while sitting there probably the master, to further confirm my first post. HTH

 

[Wikid 1]

Re: Truck creeps forward at red lights

sounds to me like the MC is leaking back through. I won't buy a rebuild MC anymore, i've had so many of them that were no good out of the box, and on top of that they just arn't the same, I dn't even wast the time anymore. the NEW one isn't that much more and can be had from any parts store and it will fix the problem.

 

[Black Knight]

Re: Truck creeps forward at red lights

sounds to me like the MC is leaking back through. I won't buy a rebuild MC anymore, i've had so many of them that were no good out of the box, and on top of that they just arn't the same, I dn't even wast the time anymore. the NEW one isn't that much more and can be had from any parts store and it will fix the problem.

Yeh I was also considering this, but due to lack of info, because we aren't told the pedal is going low leading me to think caliper since the master was recently replaced. If you had a frozen caliper the pedal would stay high and not grip the rotor letting it pull forward. Master would loose pressure and pedal would go down. Thats all I can come up with.

 

[jimbo006]

Re: Truck creeps forward at red lights

Sorry about the lack of info but the pedal will slowly sink to the floor. After it starts to creep forward I have to put a little more pressure to get it stopped again but it will still start to creep forward again in a few seconds but the pedal feels like it is staying firm, but when it starts to creep forward I do have to apply more pressure to get it stopped again. The master cylinder I bought was a brand new one and I bench bled it before installing it on the truck where I then bled the rest of the brake system. The master cylinder was replaced about 10 momths ago. The sound I hear could be described as a quacking sound...I took the check valve out and blew through it, it would let air through one way and not the other but I don't know if I am able to blow through it hard enough to replicate running conditions in the engine.

 

[QUICK STORM]

Re: Truck creeps forward at red lights

replace the check valve first, before you start throwing more parts at it. could be a leaking booster

 

[rxsyclone]

Re: Truck creeps forward at red lights

My Sy creeps pretty bad also. I know I need to bleed my lines, but my truck also idles high so I'm sure that doesn't help either. A couple times a week it will also hunt really bad, could it be a bad TPS or something?