[JohnWR]

This is on my to-do list, but other forum members have mentioned it can be a daunting task. I have a tech 2 clone and have been pondering the do-ability of this “upgrade”.

Have you experienced the problem? My RL does not have the pump, but I always let the engine idle long enough to drop out of fast idle before moving and have never had an issue. I have also read that the tendency of manual transmission drivers to "blip" the throttle before engaging the clutch helps to make this mainly an automatic transmission problem.

 

[JohnWR]

In heavy but highway speed traffic i have been in boost when a non attentive driver merges right in front of me causing my foot to instantly swith from throttle to brake.
Brake pedal stuttered towards the floor.
Very disconcerting.

I thought that there was a check valve in the line to the booster that would hold enough vaccum for a stop in that situation.

 

[JohnWR]

@HHGadget running out of vacuum would have the opposite effect - a rock hard pedal that doesn't seem to do anything.

You can replicate the experience by pumping the brake a few times with the engine off to bleed off residual vacuum. Not only would that replicate the pedal feel, but it'd be indicative of how many pedal presses you have stored up in the booster.

@JohnWR this thread should probably be split.

Thread has been split.

Wasn't there a program change made to use the ABS system to generate braking force in lieu of vacuum?

Pedal chattering down does sound more like ABS activation, however. Was this before or after installation of the big brakes?

 

[JohnWR]

Both before and after.
Definitely more noticeable after though.

I have had to go from wide open throttle to full brakes only once or twice. I am ever so happy that it is not the regular occurence that it seems to be for you.

 

[JohnWR]

As @phil1734 stated, lack of vacuum typically causes a very hard pedal, since you have no assist to apply more force to the master cylinder. In the case of the missing vacuum pump, however, I recall a "fix" that reprogrammed the system to use the ABS system to help apply force in the absence of vacuum. Do you initially feel a very hard pedal, or does it immediately sink to the floor?

Pad knockback would, I assume, be the result of excessive runout of one or more brake discs, and should be measureable. Wouldn't it also be present in normal braking, though?

I haven't experienced it with a brake master cylinder, but I did have a clutch master cylinder that got a small piece of - something - in the port that introduces fluid to it from the reservoir. First occasionally, then more often, there would be insufficient fluid in the cylinder to actuate the clutch. This seems unlikely for the brakes since there is so little fluid flow during braking, but it is something that happened. I also do not think it would result in chattering, but would simply be a soft pedal.

Given that you say it is worse after the installation of the bigger brakes it could well be normal ABS action, exacerbated by more brake.

 

[JohnWR]

I dont have a vacuum pump on either car.

But your other car is an NA, so doesn't need one.

 

[JohnWR]

This is very interesting. I wonder if the ABS system can be calibrated via the Tech2 Clone?

Probably not. Most of the modules are not programmable with the Tech2, it is used to select between the options that were available for that specific car. In the case of the ECM there was no option for bigger brakes, so there is nothing different to select.

 

[JohnWR]

I'm not sure what exactly you would calibrate anyway.

If true, the continued dropping of the pedal would indicate that the unit isn't able to keep up with returning the fluid that the dump valve releases. Given that dump valves will not be capable of flowing any more fluid than originally designed for and the pump is supposed to be able to keep up with this amount of fluid, this would indicate to me a partial failure of the ABS pump or a blockage in the manifold.

My thought was that, even though big brakes don't really add stopping power, changing the characteristics of the hydraulic system could change the system response.