Brake Problem Help?

[AluminatorSnake]

Recently I have been noticing a funny feeling in my brake pedal. When I cruise with the AC/Defrost on and hit the brakes at a moderate rate, like coming to a stop, the pedal will feel mushy and gritty for a second, then firm up. Almost like an ABS kicks in. It doesnt do it with the AC/defrost off, only when its on.

Anyone else have this problem? What was your solution? Or where should I start looking to trouble shoot this? Ive never had this feeling happen on any car, and not real sure where to look, so any help is appreciated.

Thanks

 

[Myersprostatus]

does the pedal ever drop randomly? or does it have a shutter effect? A mushy pedal or soft pedal along with a pedal dropping could be cause by air in the system, a master cylinder with an internal leak, or the Hydraulic Control Unit(ABS/traction control).

The first thing I would do is check the system for leaks at the brake lines/calipers.

Second to test the master cylinder you have to take the lines off and plug the ports. Dont do it with your fingers otherwise brake fluid will explode in your face. You literally have to have a plug that threads into the ports. You ports wont take a typical bolt as the threads are different. You need some plugs from a bleeder kit or what I did was go to a junkyard, find a mustang, cut the lines/threaded ends that go to a mastercylinder port and use them as plugs for your mustang.
Once you have the the ports blocked off on your master cylinder apply the brakes. If you brake pedal maintains height and doesn't move you know your master cylinder is working and doesnt have an internal leak.

Last but not least to test the HCU there will be two rubber plugs on the Hydraulic Control Unit. The HCU is located right behind the Radiator/fan from what I remember and has brake lines coming from it. A leaking Dump Valve inside of the HCU will cause your symptoms. I have never replaced a HCU on a mustang, but I have on a f150. I could not find the location of the rubber plugs or the amount of play allowed for the valve for the mustang HCU in the ford workshop manual, but I take it, its similar to the F150. Anyways the test asks that you stick a paperclip end or small screwdriver into the port that was being covered by the rubber plug. With the paperclip in the port have someone apply the brakes. If the paperclip or small screwdriver moves out 0.25in or more you know you have a bad HCU.
If it does end up being the HCU it is a fairly pricey item and you have to use an IDS or NGS(ford scan tools) to bleed the HCU. I typically manually bleed the whole system, then bleed the HCU using the NGS/IDS, then bleed the whole system again just to get out any air bubbles.
The HCU is kind of a pain to replace and the fittings on the lines can crossthread really easily.

Anyways I hope this helped. Let me know if you have anymore questions. I have full access to Fords workshop manuals and various repair tests.

 

[AluminatorSnake]

Sorry I just got back to this thread, thanks for the info.

Ive never had the pedal drop, just a mushy gritty feeling for a short moment in travel at certain times.