Pedal Goes Completely Stiff If You Push It Down Too Fast But Otherwise Brakes Work Great.

[sonicz]

On my 93 R33, the brake pedal has a really bad habit of knocking you back or holding you in a spot if you apply pressure to fast, for example in a panic brake situation. This is BEFORE you even reach the initial bite point of the brakes. It has resistance with zero actual braking force and its totally dependant on how fast you apply pressure.

Lets say if you try slam on the brakes while you are driving down the road and see something, the pedal will get rock hard, completely keeping the pedal from getting to the point where the brakes bite. It will hover here no mater how hard yo press, the harder you press the stiffer it gets. After a few seconds it will start to move down again at its own rate (kind of like a shock absorber) and get into the point where the brakes bite normally. IF you ease the pedal slowly down during the first parts of the movement, getting past the initial movement slowly, you can then slam it down and it wont lock up on you.

What exactly is going on here.

The brakes are strong and dont have a lot of pedal travel otherwise.

Edited November 18, 2014 by sonicz

[Ben C34]

Something is f**ked.....

Are you able to pull the master cylinder and booster off?

If not take it to a brake place asap. That's some dangerous shit right there.

[GTR_JOEY]

Does it feel like the brake booster isn't working?

As in does the fault described feel the same if you try to brake with the car off?

[GTSBoy]

This sounds like a serious MC fault.

[GTR_JOEY]

Maybe the cylinder is jamming?

[Ben C34]

Doesn't matter.

Has to be taken apart and fixed regardless. It could be the pivot point.

Doesn't change the fact it's extremely dangerous and a chat on a forum won't fix it.

[GTR_JOEY]

This is true, I wouldn't be driving it lol

[jake26]

Take it to a mechanic ASAP

[sonicz]

Maybe it came across a bit too dramatically. Its not scary bad, its just a bit strange and the brakes otherwise feel like they are working easily and normally. The car recently passed rego and a average driver they probably would not even notice it. I guess I could describe the brakes using the common "mushy" or "spongy" terminology, but only at the start of the pedal travel and then they bite down really well and strong. Normally when brakes a spongy they are spongy throughout.

I am going to change the brake fluid as I think it could be air in the brake fluid making things mushy. Use any Dot 5.1 grade right? Its ABS equipped.

The perfect analogy would be the shock absorber on your computer chair. Take the chair all the way to the top, then hold the handle which allows you to go down again and sit on it fast from a height whilst holding the handle. You'll notice it wont let you just slam the chair straight down, but rather resist you plenty initially, then ease of holding you back and let you go down at a faster pace

Edited November 20, 2014 by sonicz

[sonicz]

Just changed the pads and fluid and its much better now. Looks like it was related to air and water in the old possibly never changed brake fluid. It still has about 30% of that stiffness left however so I wonder if it was done right.

[GTR_JOEY]

You completely flushed that old fluid out before you put that 5.1 in didn't you?

[SargeRX8]

You'll probably need two bottles of fluid to ensure you get all the old shit out. Water is pretty bad in a hydraulic setup.

[GTR_JOEY]

That and you cant mix synthetic based fluids with mineral based

[sonicz]

Yea most of the old stuff out, though obviously some residuals would be left in reservoir tank and lining of components. I think the order was rear left rear right front left front right.

I had/have no idea that you cant mix synthetic and mineral based. Have no idea what the previous owner had. I used dot 5.1 from supercheap.

How come all the "good" stuff seems to be dot 4? Is that ok to use? I think dot 5 is the one not to use, correct?

I still feel like there is air in the system somewhere that gets pumped around, maybe stuck in the ABS unit, how on earth do you bleed that.

Edited November 25, 2014 by sonicz

[GTR_JOEY]

Dot 4 is fine mate, wasting your money on 5.. You can't use dot 4 when the system recommends dot 5 as the seals won't like it..

But you can go away the other way as far as I know,

That order was correct

[scotty nm35]

I still feel like there is air in the system somewhere that gets pumped around, maybe stuck in the ABS unit, how on earth do you bleed that.

I had a lot of issues bleeding our brake systems, until I bought a vac bleeder. First time every time now and it's a one man operation.

Sounds like the seal is shot in your booster. Just grab another. I have an auto one here... (The manual booster is smaller I am told, to fit the clutch slave cylinder.)

[sonicz]

Thanks for the tips guys. I don't think anything is wrong with my seals or components, at first I suspected it may be the case, but now I'm pretty sure it was just air in the system as they are a lot better now.

At the moment its ok, but still I don't feel it was done 100% right. IT seems the Skyline brake system is not an easy one to bleed, something in the design makes it that if you're not spot on and know exactly what to do you will have problems, certainly more so than other cars like mazdas I've owned.

Scotty, any info on where to pickup a cheap vac bleeder?

Edited December 1, 2014 by sonicz