1980 K10 Brake question

[flatford39]

I had just finished changing my complete exhaust system on my 1980 K10 and I got behind the wheel to start the truck up and as is my custom I had my right foot on the brake while I was listening for leaks when all of a sudden the pedal went to the floor. This is while the truck is in park and idling. When I shut the engine off the pedal resumes its normal position which is pretty high indicating a lot of brake available,but once the engine is started and I apply pressure the pedal goes to the floor. I don't see any leakage anywhere and the MC is full. Is this vacuum related or do I have a bad master cylinder???

thanks for any advise.

 

[bucket]

If there's no leak it must be a bad master cylinder. With the truck not running, if you push really hard on the pedal does it still not go to the floor (it would be slow if it did)?

 

[flatford39]

If there's no leak it must be a bad master cylinder. With the truck not running, if you push really hard on the pedal does it still not go to the floor (it would be slow if it did)?

I just went back out there and put both feet on it with the engine off. The pedal went to the floor rather quickly like it does when the engine is running. I suspect it's the master cylinder now. I am really a pretty lucky guy here as this happened in my driveway and not while I was driving.

Thanks for the help.

 

[Mr Clean]

Sounds like a master cylinder, do you have power brakes? Could be booster.

 

[flatford39]

Sounds like a master cylinder, do you have power brakes? Could be booster.

I do have power brakes. Is there a way to determine if it's the MC or the booster??

Thanks.

 

[bucket]

Pretty sure that the only problem a failed booster can cause is a lack of power assist or a funny hissing noise when you apply the brakes.

 

[highdesertrange]

booster would not cause this. master is bad, internally bypassing. highdesertranger

 

[flatford39]

Thank you for all the reply's. I have ordered my MC from Rock Auto and in the meantime I am trying to loosen the air bleeders on the rear brakes and front calipers. I have been soaking them every night with Kroil for about 4 days but they don't seem to want to budge. I will be patient and continue to apply the Kroil but are there any tips out there on how to get these loose without breaking them off in the wheel cylinder or caliper??

Thanks again for any help.

 

[bucket]

Lightly bop them with a hammer, direct hit straight down on them. Not a hit to the side of them. You can also hit the area around the base of the bleeder, with a hammer and punch (striking the caliper/wheel cylinder, not the bleeder itself).

 

[flatford39]

Lightly bop them with a hammer, direct hit straight down on them. Not a hit to the side of them. You can also hit the area around the base of the bleeder, with a hammer and punch (striking the caliper/wheel cylinder, not the bleeder itself).

Bucket,

Thanks for the advise. This did work and I finally got the last one open this AM. It was the right rear. Now on to installing the new master cylinder.

Not sure which penetrate did the job but I was using Kroil at first and then moved to PB Blaster. They started breaking free after the switch of penetrates but not until a lot of light tapping as you advised.

Thanks again.

 

[bucket]

Glad it worked for you!