Brake pedal stuck

[Furetto7]

I am hoping that the older and wiser heads here can help me with a problem. Mid to late 1986 M998, has had a couple of the MWO upgrades done but still has the original engine and transmission, less than 10k on the odometer, full military configuration.

It hasn't been driven much as we initially thought the parking brake was stuck, after fixing a number of light issues related to bad grounds I now find that the brake lights are on all of the time, the pedal is almost to the floor and will move less than an inch, I have tried pulling the pedal back by hand with no luck. I am thinking either the Hydrobooster or the master cylinder, before I start tearing the brake system apart and perhaps spending money needlessly I am hoping someone can point me in the right direction. I have been trough the manuals available on this site and know how to replace the parts but don't know which parts to replace, I would like to avoid tearing the system apart and then have to wait for parts to arrive. As best as I can determine the vehicle sat outside with the canvas up and hood closed for several months to a year, it started with just a little shot of starter fluid on the first start up and hasn't caused any problems starting since.

Thanks,

Furetto7

 

[Retiredwarhorses]

First off...never use starting fluid on these motors...now, onto you your issue.
pull the cotter pin on the lollie pop on the hydro boost and disconnect it. See if your pedal moves freely, there is also a return spring on the pedal, make sure it's ok and connected, If that is ok, your hydro may have failed.
you will also notice that there is a brake switch with an "L" lever that goes to the pedal, that is where the rear lamps get there voltqge from when the pedal is depressed....those can be problematic when they get old.

 

[Retiredwarhorses]

Oh, could also be a failed master cyl...how are the brakes them selves? Locked, work ok?

 

[Furetto7]

Thank you both for the quick replies. The ether was used by one of the mechanics at the depot that we picked the M998 up from to start it. The pedal is stuck in the down position with the service brakes applied, I have tried pulling up on the pedal without success, I disconnected the brake light switch and it tests out ok. I am going to separate the Hydrobooster from the pedal assembly later today, my fear is that either the master cylinder is so badly corroded that it is frozen or something has gone wrong in the hydrobooster, master cylinders I know how to repair/replace and from what I can tell from the TMs the hydrobooster is a replace only option. Thanks again for the information.