today i was driving around and suddenly I some how remembered about a few people talking about having to push their brakes twice, the first time would be soft and the second one would be hard and would grab a lot harder. I tried this in my car and indeed found that the second time I pushed my brakes, it did grab quit a bit harder. Could anyone give me any links to that disscussion or tell me why it is doing this? (i searched but could not find anything)

thanks

 

Pumping the brakes once before you hold them will pump more fluid into the lines providing you with a sort of preload on your pedal. Over time, with the brakes relaxed, the fluid crosses back through the master cylinder into the reservoir, balancing out the pressure. When you depress the pedal again, you pressurize the fluid between the cylinder and the calipers. Adding additional pumps allows for that negative pressure from the extended brake pistons to equalize by pulling more fluid into the cylinder, starting the brake stroke with more fluid between the master and the calipers. I think I just went in circles with that, but let me know if it helps.

 

my guess would be the first time you press on the brakes it builds the pressure up, so the second time when you press on the brakes the brake fluid has already been compressed and you get more force. kinda like how ss brake lines help.

 

ahhh, so its not something wrong with my brake cylinders or capilars

 

Air in the lines is said to make them soft on the first application and harder on the second. I believe the first pump compresses the bubbles, and then a quick second pump doesn't have to travel as far to compress the bubbles, because the brake fluid doesn't get pushed out fast enough to completely conteract the pressure from the first pump. Air in the lines generally will make the pedal softer all the time, and isn't something you want anyway.

Some people claim that a good thorough bleeding will get all the air out and that the second application should feel just like the first. But I don't believe that because I've never seen a car that didn't have a firmer pedal on the second pump. I believe that even if there is no air in the system, the second pump will be firmer. I think it has to do with the first pump pushing the pads onto the rotor and the second pump doesn't have to use as much travel to get the pads tight against the rotor. Given some time in between pumps and with the car moving, the pads will retract a bit. When you do the second pump right after the first one, the pads don't retract as much, so the pedal is firmer.

-Max

 

my car is feeling like this as well... but i dont recall of this happening before... so ill say something is wrong with mine... maybe i need to change the brake fluid??

 

There shouldnt be a big difference, maybe a little. If it is a night and day type of difference in firmness then a really good brake bleeding job is in order to get the air out of the lines.

 

Ever since I own my FD, I'd say 7 years and so I had that happening, very lilte noticable though, maxcooper's post tells you exactly why.

 

Oh yeah, I forgot to mention that in the begining when I first noticed I bleeded my brakes few times but it realy didn't help much, the most you will notice that is when your brakes are at the end and more fluid will need to be pushed.

 

hmmm... any write up on how to bleed the brakes fluid...?