Moodster

These are all just spitball ideas to try to determine the real problem and I appreciate everyone's input. I looked into a return spring today and there is nowhere to put it. The brake pedal bends around the steering column and that area is really tight. If indeed the pushrod length is the problem then a replacement booster from the same supplier will likely perform the same. However if it is the valve then maybe it is just a defect in this one booster. These ideas help me to determine the right direction. Thanks everyone for the help.

Wow, that is one of the best explanations I've read for how a vacuum booster works. I was beginning to think I would never understand what was going on. To test for the pushrod to the pedal being too long, do you think I could put a washer between the booster mounts and the firewall? The pushrod is non-adjustable on that side so short of cutting it into and threading it with some rod, jam nuts, etc I don't have any way to change that length.

Unfortunately there is no pushrod adjustment between the pedal and the booster. When you set the pushrod length going to the master cylinder does it matter if you have vacuum being applied to the booster? Would there be any harm in purposefully making the pushrod too short to verify that isn't the problem? I used this tool when I changed the booster to be more accurate with the pushrod adjustments: https://www.npdlink.com/product/tool-power-brake-booster-rod-length-adjustment/222186?backurl=search%2Fproducts%3Fsearch_terms%3Dbooster%2Badjustment%2Btool%26top_parent%3D200001%26year%3D1969&year=1969

This is the new booster I used: https://www.npdlink.com/product/booster-assy-power-brake-midland-ross-style/215184?backurl=search%2Fproducts%3Fpage%3D1%26search_terms%3Dvacuum%2Bbooster%26top_parent%3D200001%26year%3D1969&year=1969 Here is a picture in the car:

I disconnected the vacuum booster and did my typical drive into town, picked up some groceries and back. Lots of different braking - mostly easy but a few quick, hard stops. At first I thought I might not be able to do this since the pedal was so hard to press down however I got used to it. The issue did not appear the entire time. Then I got home, quickly hooked up the vacuum booster, started the car and the issue was not present at first. I took it for a quick drive around my neighborhood and it showed up within a few hundred feet of my house. One other thing I noticed is that the brakes do appear to be slightly dragging even when the brake booster is working. That's because I found the car rolls so much easier and better without the booster hooked up. Am I safe to conclude the brake booster is bad?

Thanks guys, this is all good feedback and contributions - exactly what I needed. It rained here all day yesterday however it looks nice today. I'm going to do some of the tests mentioned and let you all know the results.

I’ve been suspicious of the master cylinder all along but I’ve never been able to find a problem with it. I bought a second new one just to be sure. All three master cylinders have shown the same issue. My original one worked for half a dozen years or more. I think it could still be the culprit but maybe aggravated by some other condition.

That’s a good idea - I’ll try unhooking the vacuum booster and see if the problem is still there. I’m also going to try insulating the booster and master from the heat and seeing if that makes any difference. Thanks everyone for the help.

My car originally came with drums all around but I upgraded to Kelsey-Hayes disc brakes on the front. These were the same disc brakes as the factory used on early Mustangs that had this option. I drove the car for several years without any issues but about two years ago we had our house renovated and I put the car in storage. My problems with the brakes started after that. First let me say that I've replaced just about everything in the brake system. Two new master cylinders, new brake booster, new calipers, new front/back rubber lines, new lines going into the master cylinder and I've bled the system countless times. My proportioning valve and brake splitter block is about eight years old and I bought it from a reputable part supplier for a stock disc/drum 69 Mustang. Brakes work fantastic when I first drive the car however once the engine gets hot, the brake pedal stays down when I press it. A few clues: 1. I don't appear to have any brake fluid leaks. Vacuum booster is dry behind the master cylinder. Discs and drums are dry, no visible movement in the level of fluid in the resevoir. 2. Bench bleeding the master cylinder shows no leaks and the piston return spring is very strong. 3. When the brake pedal sticks, I can easily pull the pedal back up with my foot and the car moves freely again. 4. I do have ceramic coated headers and the vacuum booster/master cylinder does get hot but I can still touch it. 5. If I turn the engine off when the brake pedal is stuck down, it will slowly release. 6. If I leave the engine on with the brake pedal stuck down - jacking the car up shows me that all four wheels have the brakes engaged. 7. My vacuum gauge shows 13-15 inches of vacuum to the booster when hot and 16-18 when cold. 8. On the latest master cylinder, I set the length of the pushrod to the master cylinder using a special tool. I found that the pushrod was a lot shorter than it needed to be. The rod going from the booster to the pedal is non-adjustable. 9. 69 Mustangs didn't use a pedal return spring. 10. I haven't replaced the rear drum wheel cylinders. For all I know they are original however they don't leak and they do work. I've been ruling them out since the problem appears to lock all the wheels. 11. If I start the engine and let it warm up in the driveway then I can repeatedly use the brakes without issue. The vacuum does appear to be higher and steadier though than when I get back from driving and the brake pedal is staying depressed. I suspect the problem is that the booster is not getting enough vacuum. However I don't understand why slightly low vacuum would cause this behavior. The booster is new but I had this same problem with the old booster. I'm at a loss, I really need some insight into the problem so that I can just stop buying so many parts. Thanks so much for any help. David