Cause for having to double-pump brake pedal to get good performance?
 

Over the past few months, my 986 brake pedal had became increasingly spongy. I bled the brakes and found bubbles in the two front calipers (the two rears were fine). After bleeding, the pedal was nice and hard with the engine off. When I started the engine, the pedal became softer, but was still harder than before bleeding. On the road, while things had improved, I still felt the pedal was too spongy and found that I could get much better braking performance by double-pumping. It doesn't matter if the brakes are hot or not, double-pumping always gives better braking than a single pump. Rotors and pads are in great shape, so it's not them.

Two questions:

- Does this sound like I still have air in the lines or could this be a master cylinder problem that is requiring the need to double-pump?

- Is the fact that the pedal softens when the engine starts normal and expected (due to the vacuum assist) or should it remain as hard as when the engine is off?

TIA!

the pedal will always be softer when the engine is on, but not spongy as you describe.
when you bleed the system, did you also flashed it?
if not, when was the last time the system was flashed?

double pump always means there is some kind of trouble with the brakes, usually air in the system the question is where is the air coming from? you might have a leak in the lines somewhere. Do a thorough search for any signs of brake fluid. even inside your frunk. Do you have PSM ? You might also have moisture in the system. did you flush the entire system?
On your second point does the pedal actually soften or does the pedal move closer to the floor?

Besides air in the system, perhaps you have a failing component like a brake booster or master cylinder. Check for leaks.. Get it diagnosed by a local shop.

How old is the brake fluid by the way?

Thanks everyone. Here is some more info:

- This is a 99 Spec Boxster race car with a completely stock brake system (with Performance Friction 06 pads)
- The car does not have PSM
- The brake fluid is only two race weekends old and is ProSpeed RS683 – premium racing brake fluid
- The rotors and pads are in excellent shape
- There are no leaks anywhere
- The pedal is nice and hard when the engine is off and does not move when constant pressure is applied
- The pedal softens when the engine is started (as seems to be expected) and the pedal does not move any further or go to the floor when constant pressure is applied
- While driving, a double-pump clearly creates more stopping power than a single pump

My primary suspicion is that I did not get all the air out when I bled the brakes, or maybe the fluid is contanimanted and needs to be fully flushed out. But I was curious if these symptoms could also indicate a possible master cylinder problem (or brake booster problem)?

I can't remember exactly, but did you bleed the clutch? some of these hydraulic systems use the same fluid for clutch as brakes.

The issue, like you say, is essentially a loss of pressure, or firm pedal, once the car is started and the vacumn systems are started. This is why I would think a failing brake booster or MC could be at fault. These are, now, 13 year old parts. They don't last forever.

If you truly believe it to be air trapped in the system then keep bleeding.. but how/when did you introduce air into the system? Did the bleeder run dry? did the resevoir go dry while bleeding?

NEW INFO - did not know under what condition you experienced this. most likely the brake fluid it boiling (or nearly)and yes you will get a better pedal on the second pump. I'm not familiar with prospeed but if the 683 is an indication of the boiling point them you should be OK . Having said all that, it is also possible the master cylinder is starting to go but if I remember right I had to pump mine more than twice to get a brake on the track. I would conside installing a new master cylinder ( cheap insurance)

Update: I got a suggestion that the wheel bearings could also be a cause, but I checked those last night and there is no play at all, so it's not them. This weekend, I'm going to flush the fluid out and see if that fixes it. If not, it's on to the MC and if that doesn't work, the brake booster. I've never bled the clutch (good idea), but the clutch works perfectly right now.

Yes the ProSpeed RS683 is a premium fluid with a 683 boilinng point and is very competitive with (and most think even better than) SRF. Really doubt I got anywhere near boiling it. As for air, anytime you flush or bleed the brakes, you run the risk of introducing air, so I'm going to try to elminate that before I go buy parts.

Years ago I had a 1602 BMW. I was doing the brake double pump. Rebuilt the master cylinder and problem went away.
Not sure how wheel bearings could have anything to do with this!!

Only thing I can think of is if there is a vibration in the wheel (from a loose bearing) that it will make rotor not track true and that vibration or ocillation will push the pads back into the caliper more than usual and they will need to travel a longer distance back out to make contact full with the rotors.

Did anyone find out what the issue was on this?? I have the same issue with my brakes. I installed Boxster calipers on a 1970 911. Need to lightly pump the brake with my left foot on the straights to have a firm pedal. Pedal stays firm in the braking zones but goes soft again after anything longer then a 10 sec interval off the brakes. Flushed the fluid twice. pads are decent and rotors are near new.

I was thinking rotors or wheel bearing as well? But they seem true.