Brake feel question

JayJay · Post by **JayJay** » Mon Jul 02, 2012 4:05 pm

I've wondered this for a while but I haven't asked.

Sometimes as I'm braking and I'm gearing down (i.e. the RPM is changing the brake seems to depress/become softer. Just what could cause? It's quite annoying!

Also, has anyone else noticed that the brake fluid reservoir cap has an arrow on it that points to "front"? I didn't realise the cap had to be put on a certain way

Post by **Merlin** » Mon Jul 02, 2012 4:13 pm

I think you have described the same problem that Vanzep has.

Supercharged 無限MUGEN BB1 Profile · Post by **nucleustylzlude** » Mon Jul 02, 2012 4:56 pm

Is this something to do with the brake booster? Just a thought? Although I don't recall this on mine, once upon a time when I last drove it. And I tend to engine brake alot to save brakes in general commuting.

Supermarine Blues · Post by **Supermarine Blues** » Mon Jul 02, 2012 5:03 pm

Not entirely sure what you mean; is the pedal slowly sinking (ie fluid leak past the seals) or does it feel that the assistance increases with more RPM on overrun & increased manifold depression?

If the latter, your servo valving may be getting tired. It shouldn't happen like that.

Post by **vanzep** » Mon Jul 02, 2012 5:14 pm

JayJay wrote:I've wondered this for a while but I haven't asked.

Sometimes as I'm braking and I'm gearing down (i.e. the RPM is changing the brake seems to depress/become softer. Just what could cause? It's quite annoying!

Also, has anyone else noticed that the brake fluid reservoir cap has an arrow on it that points to "front"? I didn't realise the cap had to be put on a certain way

Never noticed the arrow on the cap

Garage said mine was master cylinder probably
Mine only gives soft pedal feel initially after start up then a few pumps of the pedal and it firms back up.
Ive either got used to it or its healing itself

either way i am noticing it less and less.....

Your brake problem sound different.
As i said its probably master cylinder or brake fluid has gone off - i need to flush the old brake fluid out entirely and fill up with some good quality fluid. Bear in mind i ve replaced all four calipers and brake lines.
This is next on my list of things to do so i will let you know how i get on if it helps any.

Post by **Donald** » Mon Jul 02, 2012 5:58 pm

I have the same thing as vanzep.

JayJay · Post by **JayJay** » Wed Jul 04, 2012 9:27 am

I've just replaced all the fluid

I can't remember the exact sequence of things at the moment

But basically as the RPMs change the pedal feel as I'm braking goes from solid to softer. It doesn't affect the braking itself as far as I can tell, just the pedal, and there is no fluid leaking (the pedal doesn't sink).

I figured as the pedal feel is connected to the RPM it must have something to do with some brake system unit that is connected to the vacuum system.

Post by **lewd lude lover** » Wed Jul 04, 2012 12:33 pm

nucleustylzlude wrote: And I tend to engine brake alot to save brakes in general commuting.

because changing the clutch is so much cheaper than the brakes

Confused · Post by **Confused** » Wed Jul 04, 2012 1:02 pm

Sounds like the brake servo isn't holding vacuum correctly.

When you brake, there is a large diaphram within the servo which is activated by vacuum (which is generated from the engine) and there's a 1-way valve to stop the vacuum from depleting.

As you engine brake, the engine revs increase, which in turn is generating more of a vacuum. If the vacuum that the servo requires is disappearing, this increase in vacuum generated by the engine will bring the servo back up to it's proper operational parameters.

The first thing to check is for a leak on the large bore pipe that's feeding it from the inlet plenum to the servo. There is also a 1-way valve on this pipe, check this for correct operation too. It should allow airflow towards the engine, but not away.

JayJay · Post by **JayJay** » Wed Jul 04, 2012 4:50 pm

Top man there, Confused. I'll have a look.

I remembered just what happens - say I'm going pretty fast and I slip the car in neutral going down a hill. I start braking, then slip the car in to a low gear. As the revs rise the pedal feel softens. Not a lot, it's not like it becomes nothing or keeps softening, but it's perceptively softer. So as the revs rise the pedal feel becomes noticeably softer.