Soft brake pedal on first push and then stiffens up (when engine is running)

[crutch]

Recently picked up my first FJ60 and brakes were seized up, replaced most of the system including MC, rotors, pads, calipers, brake shoes and wheel cylinders. Pressure bled the system a couple of times to flush fluid (2.5L). One thing I noticed when replacing the old MC, it was leaking in the back against the booster and I suspect fluid got into the MC.

When test driving, the first push on the pedal has a lot of travel almost to the floor but subsequent pedals stiffen up right after. This seems to be only when the engine is running. Brakes work great even on that first push you just have to go real far to the floor. There is also a hissing sound that is well pronounced when pushing pedal.

I was hoping to lean on your expertise before purchasing a new brake booster just to make sure. Here is a picture of the booster after I removed the MC.

Thanks for any insights!

 

[Skreddy]

Did you adjust the pushrod between the booster and master cylinder? Needs to be done whenever either is replaced and is often overlooked. I’d start there. Then, depends how tight you adjusted the rear shoes. If they are too loose, your first pump fills the wheel cylinders and the second pump presses them against the drum.

 

[The Machinist]

Recently picked up my first FJ60 and brakes were seized up, replaced most of the system including MC, rotors, pads, calipers, brake shoes and wheel cylinders. Pressure bled the system a couple of times to flush fluid (2.5L). One thing I noticed when replacing the old MC, it was leaking in the back against the booster and I suspect fluid got into the MC.

When test driving, the first push on the pedal has a lot of travel almost to the floor but subsequent pedals stiffen up right after. This seems to be only when the engine is running. Brakes work great even on that first push you just have to go real far to the floor. There is also a hissing sound that is well pronounced when pushing pedal.

I was hoping to lean on your expertise before purchasing a new brake booster just to make sure. Here is a picture of the booster after I removed the MC.

View attachment 4027911

Thanks for any insights!

When you say there is a hissing sound, is it continuous while the pedal held? If so the diaphragm in the booster probably has a leak and would need to be replaced.

Good news is that they are fairly cheap and you have already done all of the hard work by removing the master. It would not be a bad idea to just replace it while it's all apart.

 

[crutch]

Thank you all for the feedback. I went back and adjusted the rear drums and that was the ticket! Ran some test on the booster and is holding vacuum. Thanks again!