On my 74 2.0L with stock brake setup, when I hit the brakes the pedal can feel soft and if I let up on the brake and then depress again right away, it then becomes very firm.

Note that by "soft" feel on the pedal when first depressing the brakes does not mean that the car can not be stopped. Also the pedal is not "soft" in the sense that it depresses all the way to the floor.

I seem to remember an old Datsun 240Z I had with similar braking behavior. Is this just behavior of 1970's braking systems or do I have air in the system or a faulty master cylinder?

This sounds like you have air in the system somewhere. I would first bleed the brakes and see if you get any air out of it.

Check your rear caliper venting clearance. Then check for sticking pistons.

Does it behave that way immediately after starting it, or after driving for a long time without depressing the brake.

I had a case where after driving on the highway and not using the brakes for a while, I would press the brake and have almost no brakes until I pumped them.

That problem was swollen liners on the flex lines on the front. They would not let the pressure relieve and the brakes would drag. Then they would get so hot that the brake fluid would boil and push the fluid from the caliper.

New flex lines solved that problem.

I had this on my first 914. Could never bleed it out of the system. But it went away when the master cylinder was replaced. I have no logic or reason to explain how it was happening but I have to think that it was a sign of bad things to come!

By chance: did you replace the flex lines at the same time?

The seals in the master cylinder are going bad. Cold weather can make it more apparent than when it's warmer- when you pump it the second time the seal flares out and doesn't let fluid bleed by.

Set the rear venting clearance
Then
Bleed the brakes, rears first, then front
Use the top bleeder if you have two.

Then drive the car, bed the brake pads.....meaning heat up the brakes, go 0-60 and then back to 0 about 10 times.
See PMBperformance for the tech article on how to do this.

Then bleed them again, you will have moved any air bubble through the rear brake valve so the pedal may get soft again.
Mine does every time....so this is my method, YMMV


Summary
Set rear vents
Bleed brakes
Drive
Bleed brakes


Also a great time to purge all the old fluid and change it for some new fluid.
They make red, blue and amber fluid to help you know you are done...

Nope on the lines but as noted in the post below it was very cold at the time of the replacement. It stumped me at the time so I had to think it was the master cylinder seals.

Time for a brake system overhaul. If it were my car I would pull all for calipers and kit them. You could send them to Eric if you want them pretty but if your on a budget he sells kits too. New pads are a must. I would never rebuild the calipers and keep the soft lines. They are very likely swollen inside or softened. The rubber degrades over time. The sludge will get into your calipers and dirty up the fluid. As long as your replacing the soft lines and rebuilding the calipers you better ditch the master cylinder. All this is not as expensive as loosing it coming into a corner too hot and having soft brakes.

Once your done follow the bedding instructions on Eric's site.

I think Rick is correct.

Talk to Eric, get his rebuilt calipers, replace all of the rubber lines, do the venting clearance - it's not that hard, put in a new MC, and use speed bleeders.


While you are in there, turn the rotors and put in new pads.

Not any more, well at least not "LEGAL" for use on street vehicles.

The US Government, they're here to help!

Thanks for all the replies. I have read here that many upgrade the Master Cylinder to a larger size (from the 911's)?

Stock is 19mm? 911 master cylinder is 23mm? Bolts on?

I doubt that's it.

Rebuild them recently?

Did you set the venting clearance on the rears as Rich discussed?

Spoke with ATE at SEMA. The guy recommended food coloring for the fluid dye.

Also, stock on a 914-4 is 17mm. Stock on a 911 and 914-6 is 19mm.

Duh, great idea.

I saw the writing on the wall and loaded up with a lifetime supply of ATE blue (from Eric if I am not mistaken ;-)

I also rebuilt my four corners using Eric's kits.
I have plenty of cores availble to send if anyone want to rebuild theirs before taking the system apart.
Thumbs up to Eric Shea!

Stu

No, I have not done anything to them as I acquired the car from Camp914 in early October. I have no history on the brakes, but I believe I was told the brakes (at least pads) were new. This particular "behavior" appears to have taken hold now that it is colder in the Bay Area.

Like many, I am on a budget with my teener. I certainly want to be safe but replacing/rebuilding the whole setup seems a bit much also. Of course, not rebuilding/replacing anything could cause even more damage.

I'm leaning towards:

1. Checking for air in the system (ie. bleed the brakes)
2. Changing out the flexible lines at all four corners for new ones (and bleed again).
3. Checking the rear venting clearance (not sure how to do this)
4. Changing out the master cylinder for a new/rebuilt (and larger) one from a 911/914-6.

New soft lines never hurt.

I think you have a sticking piston.

While you're installing the lines, check the pads for uneven wear. Better yet, take the pads out and check the dust boots for tears and check for a piston that is out further than the others.

Rebuilding may seem like a bit much but I'd be willing to bet you're there. 99.9999999% of brake failures are caused by rust. Pistons and bores need to be cleaned. Best to replate so the bare metal in the bores is covered.

Use the search feature. I have a thread covering the venting clearance.

Good luck.

Set clearance on the rear brakes FIRST!!!!

Then let us know how you feel.

It's really simple to do.

If you use brake line clamps (not vise grips!) you can clamp all 4 hoses and separate your system from the master cylinder. Gently push the brake pedal and if it's sinking to the floor, you've got a bad master cylinder. If not, remove the clamps one by one until you find which wheel is giving you (and us) fits. Then attack! BTW, all great suggestions in this thread that apply to almost every vehicle.

Also look under the at the pedal box and see if there is any brake fluid leaking there. My 914 has a bad master cylinder and the brakes feel very similar to what you describe. I pulled the pedal box out and about 2 Tablespoons of fluid were leaking our of the MC every time the brake was pressed.

Also earlier you said the 19 mm was stock and the 23mm was the 911 size. That was incorrect. The 914 was stock with a 17 mm MC and the 914/6 and later 911's had a 19 mm MC. Don't use the 23 mm MC it will be about twice as hard to push the brakes, everyone I have talked to thinks that the 19 mm is the best solution.

I rebuilt my brakes from top to bottom, had the same symptoms you are having.
Only thing new I did not get was the plastic lines from the reservoir that feed the MC and run under the tank. These were unavailable new, or at least I coudn't find them. So I got the best used ones I could find.

Well the only thing that leaked was you guessed it, the lines under the tank. Pin hole in one of the plastic lines.....

Tank had to come out again for the repair.

John

John, we have those.

Good for everyone to know. Also you have added all the hard lines. I had to fab all mine. It is much easier and saves so much time having plug and play.