c4 trans slipping

Hello All,
 I am a newbie in this seen. so I need some major help. I bought an 66 coupe with the 289 and c4 for  my 16 year old to have for his first car.It was a car that is drivable and had a lot done to it but well you know the story it still needs things .
I took it to the local muffler shop to have the dual exust system on it and when I got it back the transmison started slipping. Is their any thing they could have done to cause this? I am going to drop the pan and change out the filter and oil .but could the shop bent something like the vacum line? I hope I dont have to swap out this Transmission.I still talk with the former owner and he told me he never had any slippage. please help out an old Dad trying to make his son happy.  

check the vacuum hose and tube then more hose.. from the back of the intake manifold down along the top of the transmission to the back right corner of the C4 just above the pan rail where the vacuum modulator is..

the vacuum hose can get knocked off.. it can swell and leak vacuum...

when the modulator looses a proper vacuum signal..

the car will usually accelerate in first gear.. but when you take your foot off the gas.. it kinda goes into neutral...

the vacuum tube from the back of the intake has a short section of rubber vacuum hose at the back of the intake... and another short section. usually with a molded in bend at the modulator..

the modulator hose with the bend is usually 2 bucks at most transmission parts suppliers..

the modulator has to be hooked up with a rigid steel tube..  usually necked down at both ends so the steel tube transmits instant vacuum signal to the modulator..

from the back of the intake.. to the modulator.. IT HAS TO BE WITHOUT any leaks..   you should be able to apply vacuum by disconnecting the vacuum hose at the back of the intake and pump it down with a hand vacuum pump.. or even apply suction with your lips and it should not leak..  

thank you Ill give it a try .If the modulator is bad will it leak trans fluid ?  

What type of trans fluid is best to use on a 66 c4 transmission?  I hope I did not mess up mine .I put 3 quarts of type f in it with a bad trans line leak and then  its slipping bad .I dropped the pan an changed out the filter and then put Deztron/ Mecron in it and it still aint movin.I am getting ready to order a vaccume Modulator Valve . I hope that will make it work again .  

Dakotas66:
What type of trans fluid is best to use on a 66 c4 transmission?  I hope I did not mess up mine .I put 3 quarts of type f in it with a bad trans line leak and then  its slipping bad .I dropped the pan an changed out the filter and then put Deztron/ Mecron in it and it still aint movin.I am getting ready to order a vaccume Modulator Valve . I hope that will make it work again .

Type "F" is the correct fluid for the 66 C4. Take that Mercon fluid out of there (immediately)and drain the Torque Converter by pulling the plug out of the front of it. expect 10 or so qts of fluid. If you fill the unit up to the correct line on the dipstick and it's not moving you have mechanical issues with your trans (probably a bad front pump and/or worn out bands/clutches)
 

how about air checking the transmission..

http://www.youtube.com/watch?v=ucTqlOHhCxM

this may give you some ideas...   forward clutch inner seals in these are notorious for getting hard and shrinking away.. wiping out the transmission...  preventing engagement..