brake booster question

[racer951y]

So I thought I had air in my lines and had been chasing that down for a while then today I hit the brakes with the car off and the pedal is nice and firm. I start up the car and let it run for a sec then hit the pedal again. OH NO the pedal is mushy and takes a few pumps to bring up to firmness.

So my question is this is the brake booster right? I replace that and it is fine or is there still air in the lines? Thanks for the help

[Razorback1980]

I would vote air in the lines. Try a power bleeder...makes life so much easier.

[racer951y]

Originally Posted by Razorback1980

I would vote air in the lines. Try a power bleeder...makes life so much easier.

I did power bleed all 8 bleeders. nothing but good fluid came out this last time

[Razorback1980]

Sometimes I change fluid colors so I know when the old is out and the new is in. If you can pump them up and the brakes get better, then that is usually a sign of air in the system somewhere.

[racer951y]

Originally Posted by Razorback1980

Sometimes I change fluid colors so I know when the old is out and the new is in. If you can pump them up and the brakes get better, then that is usually a sign of air in the system somewhere.

But it is fine with the car off no need to pump

[SuperXRAY]

Well, it's obviously not a brake booster problem, as the brake booster NEVER touches the brake fluid.

[Pops944]

What about the vacuum line to the brake booster? Any splits in the hose or connections? Also, what about the vacuum line check valves?

[SuperXRAY]

I'm confused by all the postings of vacuum lines being a possible culprit in squishy brakes.

If everything is bled properly, I'd suggest removing the brake master cylinder and bench testing it for leaks. Many times this is easily spotted by corrosion spots under it on the brake booster, indicating that fluid is leaking from the master cylinder seals.

[Razorback1980]

squishy brakes can only be caused by air in the lines. Bad booster or vacuum leak should give you a stiffer pedal because you wouldn't have the extra help from the booster. It's quite normal for the brake pedal to be able to put more pressure on the master cylinder with the extra boost of power supplied by the booster. Because the booster puts more pressure on the master cylinder, you can press the pedal further down because the booster is helping you. That's why when you turn the car on the brake pedal goes down further. Air will compress under pressure...brake fluid will not. Air being compressed is what causes the squishiness.

[racer951y]

Originally Posted by SuperXRAY

I'm confused by all the postings of vacuum lines being a possible culprit in squishy brakes.

If everything is bled properly, I'd suggest removing the brake master cylinder and bench testing it for leaks. Many times this is easily spotted by corrosion spots under it on the brake booster, indicating that fluid is leaking from the master cylinder seals.

the main reason is because with the car off the brakes are fine and stop the rolling car on a dime but start the engine and the pedal gets mushy and crappy and the car takes a bit farther to stop

[Razorback1980]

You are doing the right things racer...and it never hurts to question everything. I would still lean toward air or bad master cylinder.

[racer951y]

Originally Posted by Razorback1980

You are doing the right things racer...and it never hurts to question everything. I would still lean toward air or bad master cylinder.

I am starting to lean to the master also so I might just buy a new one and some ATE super blue and call it good ( I Hope )

[SuperXRAY]

racer951y,

I'm not implying anything negative. I'm not always clear as to my meaning...but I simply mean that a brake booster will not cause squishy brakes. A brake booster on most cars simply uses vacuum to increase mechanical pressure, it in no way causes a fluid leak, air to enter the lines, or anything of the sort. My post was more in response to Pops944, who inferred that a vacuum problem could be a culprit.

Air can enter brakes lines via the below, in their respective percentages of occurance:

1. Improper Bleeding.
2. Water in fluid, mainly due to improper handling/storage of fluid. The water can boil, releasing air/steam into the system.
3. Master cylinder seal wear, which is more common than credit is given. However, if the car, being a Porsche, has been well maintained, is LESS likely than most American cars. Why? Silicone brake fluid compared to what most American cars use, DOT 3 or DOT 4. Both of which attract water, whereas silicone does not.
4. Piston seal wear on the front calipers.
5. Rear seal wear or complete rear brake failure, has happened on two of the cars I have owned.

In your case, I would say it's either Master Cylinder or, much less likely, front caliper seal wear.

I suppose the reason I responded was to keep you from going down the wrong path. I urge those that don't know about brake systems to read up before they submit a suggestion that is 100% in the wrong direction. Brake are a life and death ordeal, which is why I seem so passionate about the topic.

[racer951y]

Originally Posted by SuperXRAY

racer951y,

I'm not implying anything negative. I'm not always clear as to my meaning...but I simply mean that a brake booster will not cause squishy brakes. A brake booster on most cars simply uses vacuum to increase mechanical pressure, it in no way causes a fluid leak, air to enter the lines, or anything of the sort. My post was more in response to Pops944, who inferred that a vacuum problem could be a culprit.

Air can enter brakes lines via the below, in their respective percentages of occurance:

1. Improper Bleeding.
2. Water in fluid, mainly due to improper handling/storage of fluid. The water can boil, releasing air/steam into the system.
3. Master cylinder seal wear, which is more common than credit is given. However, if the car, being a Porsche, has been well maintained, is LESS likely than most American cars. Why? Silicone brake fluid compared to what most American cars use, DOT 3 or DOT 4. Both of which attract water, whereas silicone does not.
4. Piston seal wear on the front calipers.
5. Rear seal wear or complete rear brake failure, has happened on two of the cars I have owned.

In your case, I would say it's either Master Cylinder or, much less likely, front caliper seal wear.

I suppose the reason I responded was to keep you from going down the wrong path. I urge those that don't know about brake systems to read up before they submit a suggestion that is 100% in the wrong direction. Brake are a life and death ordeal, which is why I seem so passionate about the topic.

I get were you are coming from 100% let me answer you numbers really quick and may be you will see from my angle

1. I bleed 3 cans of brand new fluid though the system both the old school have a helper way and with a power bleeder. Have been a mechanic for years I got this down pat (also why I am coming to the internet as I have never seen what my own car is doing)

2. see 1 about the 3 new cans of fluid but ya never know if the fluid is good or not

3. This is were I am leaning to now as a problem I have ways used Dot 4 prestone brake fluid in the car since I have had it (about 10 years now)

4. on the pass ft brand new seals but still could be the dr ft but that side was not dragging before (or now)

5. did the rears 2 years ago when they did the same thing

I was asking about the brake booster because of the difference in brake feel between engine on and engine off that felt like me way to squishy compared to before I rebuild the ft caliper but now I think it could be the master

[SuperXRAY]

Just be glad it wasn't 3 cans of silicone, then. $8/pint or something, crazy. Took 2.5 to fill and bleed our 944.

Hope you get it figured out.

[porsche4life]

If its a vac line you ought to run down to lindsey racing in OKC and buy the whole kit ~40 bucks.

Oh and these cars are supposed to have DOT4 only not silicone or anything else.

[racer951y]

Originally Posted by porsche4life

If its a vac line you ought to run down to lindsey racing in OKC and buy the whole kit ~40 bucks.

Oh and these cars are supposed to have DOT4 only not silicone or anything else.

Yea the ABS does not like Silicone

Also I don't like the Lindseys personally as in I know Mike and Dave and I dont like Mike at all (dave is cool though) so I do not buy from them

[SuperXRAY]

Ours is an '83, which is why I said...be glad it wasn't silicone. No ABS on ours, so I replaced the fluid with silicone.

Keep us updated.