‘89 F250 4x4 w/460 and C6 transmission. I towed my travel trailer to a park. The entire brake system had been rebuilt less than two weeks before and worked great although pedal was very hard. The closer I got the worse the brakes got. By the time I arrived they were to the floor. Had it towed to a Les Schwab (Saturday and nobody else open). They replaced the booster and they were marginally better but then the transmission wouldn’t shift automatically. Brakes spongy and soft. I had to get speed high and shift manually. It downshifted hard at high speed (45 3 to 2, 25 2 to 1). My shop is at a loss on both. Sending valve body for rebuild. Think it may be rear proportional valve on brakes but can’t find one. ABS light has been on since I bought it 2 years ago. Les Schwab says too bad they didn’t work on transmission and nothing wrong with brakes. Transmission worked great prior to Les Schwab. Any ideas?

 

The C6 has a vacuum modulator that controls upshifts depending on engine vacuum. If there's an issue with engine vacuum created by human error, a booster issue or its large vacuum line, it could affect upshifts since the C6 is vacuum controlled. If the modulator valve is blown, vacuum will suck fluid into the engine's intake. Pulling the vacuum line of the modulator and if fluid comes out, it's bad. It's externally mounted at the rear of the pan and is easy to access and replace.
If the brake pedal is at the floor, I would suspect a bad master cylinder or a leak. New doesn't mean it's good.

 

Another little clue, if the modulator is allowing trans fluid onto the intake you should see some unusual smoke coming out the exhaust. The engine will try to burn that oil and it can foul the plugs and produce unusual smoke from the exhaust.

 

Vacuum leak was the first thought we had about the trans. But according to my transmission guy, the C6 doesn't use vacuum for shifts.

 

He's wrong. The modulator is a vacuum operated device. There is either throttle linkage or vacuum utilized in transmissions of that era and the C6 uses vacuum. Ask him what is the purpose of the modulator on your C6 transmission.

 

A defective vacuum diaphram unit (Modulator) can cause no, harsh or incorrect up shifts.

 

I appreciate the input and welcome any other comments. I'm going to talk to my transmission guy tomorrow and show him this input and see if we can't figure this out. After I picked it up at Les Schwab and the trans wouldn't shift properly another mechanic had looked at it. His first thought was vacuum, but he couldn't find any vacuum line going to the transmission. Until Les Schwab got their hands on it the transmission worked great. That's why I wasn't sure about vacuum. Unfortunately, I know almost nothing about automatic transmissions.

 

It is pretty scary if the dealer worked on the shifting problem and your mechanic couldn't find any vacuum line going to the trans.
There should be one there.

 

Couldn't find the vacuum line? Maybe that's why it won't upshift.

 

Before we go overboard, how about we get the trans code from the door sticker.

This truck just could have an E4OD trans in it which could explain why they can't find a vacuum line to the trans.

Some of them DID come with an E4OD.

ghostrider,
Could you please provide the trans code as shown on the vehicle door sticker?