Hello from France, everything is in the title.
My 914 2.0 1973 with nu rear calispers and rebuild front.
MC is stock and old.
Everything works fine until I need to brake hard, since then, the pedal is hard and the car continu to brake, not really hard but I'm driving like with a trailer
Thanks for your help.

CheeRS

how long ago were the brakes serviced?

sometimes the rubber lines need to be replaced... if they are old, they can swell shut keeping the pressure in the calipers and the brakes applied.

this wouldn't likely happen to all 4 calipers at once though
can you jack the car up and confirm that all 4 wheels are under brake pressure.... or possibly just one?

Yes tomorrow I will jack the car.
I will install a used 19mm 911 MC I've kept from a restoration project just to experience it.

Too tight clearance between brake pedal and mc.

Thanks for the help.
Very interesting.
One more thing, PB is more when car is under the sun.
Also, rear caliper play is 0.1mm instead of 0,2

had this on a 66 beetle on one rear wheel line looked great externally but after replace I cut it in half and the line was mush inside,hole was all but closed over.
If the car has been unused for a long time or the lines are all very old (same age) they could all be internally swollen.
Could also be that the m/c is all varnished up in the bore causing the piston not to retract fully...gives a hard pedal too.

Yes. Rule of thumb is to replace the soft lines if you can't remember or don't know when they were last replaced.

Old rubber brake hoses can act like a check valve, they'll let pressure in but not out

Yes! Have seen that more than once.

Charlie

Your 19mm MC will make the brake pedal harder and with less travel than a stock 17.

Some may prefer this, but the stock 17 in conjunction with the stock brakes is a nice, linear set, easy to modulate.

As mentioned, unlikely four hoses would swell closed at the same time.
Also mentioned, but not fully is the need for clearance between the pedal pushrod
and the MC piston.
Adjust the pushrod so the pedal moves 1/2 inch before pushrod contacts piston.

I will this morning, Danke

bushings could be sticking....try pulling pedal back,

About the pedal, I tried.

Because I like to understand what happen, I wil make change one by one.
This morning on the lift 4 wheels were free.
I set the rear calipers clearence to 0.20mm instead of 0.10 like I did first time.
Leaving the shop no PB, much better but after 3 miles, the car start to break herself, at home rear was medium but front really hot.
Tomorrow if Saint covid allow me, I will try step 2 by setting the pedal/MC clearance.
Thanks for your help.
CheeRS

While on the lift did you pump the brakes to see which wheels were dragging?

How about after you went for a ride. Did you put it up on the lift and check to see which wheels were dragging. If all of them, it would seem the MC. If individual wheels it may mean the calipers or brake lines.

Yes I pump the brake with the 914 on the lift and everything was fine, 4 wheels were free.
The 914 has a FIA passport for historic rallye, a lot of maintenance, brake line are not old (type aviation lines).
I checked play between pedal and MC.
No play, that is to say first mm brake the car.
I put some play around 1,5 cm.
Discovered a dysfonction, pedal does not return to the top herself, need a help for the last cm.
Perhaps now the car will not brake herself with the play, but I want to fix the pedal return to fully fonctionnal.
I thing I have to change the Teflon rings, right ?

At the Corsica historic rallye.Click to view attachment

I think Jeff was right

You may need to pull your pedals for a rebuild

The bushings swell when the M C. leaks


Pull the rubber boot from the mc

Rebuild with the bronze bushings kit. rather than plastic

I would start by replacing the four flex lines

May not be the case, But make sure the rotors are not warped.

I had this happen on a motorcycle, The rotor was warped and would slowly pump up the brakes until they were full on. I had to open the bleeder and release the pressure to drive away. replaced the rotor and it solved the problem.

Thanks,
I will keep my step by step strategy and will drive the 914 to my own shop (Specialized Porsche garage) tuesday, with Covid it's only permitted to exit home during your working opening time and the garage is closed for Easter.
So now we have four possible guilties :
Lines
MC
Rotor
Pedal cluster

Next check will be to see if the PB faded away with more play and alternate when I help brake pedal to return up.

(The failure came a few week ago after a track day in Le Mans, I loose break (too hot) after 10 laps.
2 weeks after that, I made a 3 days winter Rallye on show (really few snow) in Switzeland, break worked fine but I used them just a little.)

Years ago I was told that the German TUV mandated that if a car sat for more than a year, that the soft lines needed to be replaced. This has to do with the rubber deteriorating over time when not being flexed. If the brakes are used, this deterioration is slower and they last longer. Assuming that all 4 are of the same age, it can be they will all fail about the same time.

OK, I will keep in mind this is a Possible scenario.

in the other and :
I use to change DOT every 6 monthes, sometime before each historic rally or track day.
Never met a PB with bleeding front, need to bleed rear with the pedal, perhaps because of the brake limiter.
Lines are really good quality and not really old, perhaps 3 years.

I will order a set not expensive.

RRC 2015, road fix...

Ah ah, problem resolved

A while back my brake pedal was not returning like it should so being the cheap bastard I am I sprayed WD40 on the pedal cluster and the brake pedal freed up.

I can't see how a warped rotor would "pump up the brakes". A warped rotor will knock back the pads on the first revolution of the rotor. That may then require a double pump on the brake pedal to get full braking, but there is no way that I can see that it will apply pressure to the braking system over time.

Drive test with more play between the pedal and the MC, everything is OK, no residual breaking.
Pedal do not spin at the top, i have to help for the last 10mms but without help her no pb with break.
I put some grease and WD40 but the PB carry on.
Just orederd a bronze bushing set and a new spring.

http://www.914world.com/bbs2/index.php?sho...c=32578&st=

Never look under a pedal cluster

The brake pedal was not really free to move, need new busching

Now, everything works fine, brake feeling is really smooth and no brake release.
Thanks to the community.

Great job diagnosing, getting input and hard, greasy effort! Great community of experts around in this forum, for sure! Sorry to see your floor by the brake pedal, though. Next project?