Have you fitted new brake pads and are having trouble getting a firm pedal? It seems in many cases pedal fade problems at low RPM or idle are related to diesel vehicles with twin diaphragm boosters.
Why diesels? And particularly those with twin diaphragm boosters?
As vehicles have got bigger and heavier, manufacturers in their quest to meet standards and provide vehicles that stop well with low pedal effort, have in many cases used twin diaphragm boosters.
So manufacturers in an effort to stop these heavy vehicles have been fitting larger calipers with larger pistons, large diameter disc rotors and along with this goes the need for a larger bore master cylinder, which displace more fluid.
Remember a larger bore master cylinder displaces more fluid but less pressure.
Fitting a larger bore master cylinder was mandatory to displace the volume of fluid required to operate the calipers but then with a single diaphragm booster pedal efforts became too high.
Manufacturers use twin diaphragm boosters to reduce pedal effort and give more bite. The main complaint with such set ups is a fading pedal on diesel powered vehicles.
Is it a problem, and why does it occur?
To a certain degree these vehicles are over boosted. That is, the valving in the boosters is over sensitive. So while this gives great braking performance on the road it created other problems.
On diesel vehicles where the vacuum is supplied by an engine driven vacuum pump, it seems pedal travel is accentuated. We put this down to the sensitive valving in these boosters which when coupled with a vacuum source with a slower build up time than that of a petrol engine, causes the pedal to fade away if pressure is held on the pedal at idle. In many instances this problem can be mistaken for a bypass in the master cylinder. Beware ? this is often not the case.
So what is the answer?
The first point to remember, is to assess the pedal height and feel at idle and on the road before commencing work on the vehicle.
Experience has proven that vehicles with worn, sluggish vacuum pumps can cause problems. When we say sluggish it means that the pump may be making 20 inches of vacuum but it is very slow getting there. To a certain degree fitting a vacuum tank that gives the pump a reservoir to fill up can solve some of these problems. This gives a reserve of vacuum at idle when the pump is turning over slowly.
When studying the Toyota Landcruiser 75 Series, which commonly suffers from pedal fade complaints, we see an interesting case.
Through the 1990's Toyota tried a number of different set ups. Some had single diaphragm boosters that had complaints about a lack of brakes. They then changed to a dual diaphragm booster and larger master cylinder. On the latest 79 series, Toyota has gone back to a single set up.
The single diaphragm set ups work when the vehicle is new, but struggle to pull up as kilometers get higher. Aftermarket pads with less cold bite are one cause of this.
So, if you have a vehicle with a fading pedal at idle, what checks can be done?
Firstly check if the vehicle has a hard pedal without the motor running and the vacuum exhausted. If so the problem is not the master cylinder but quite possibly vacuum related.
Our view is that if the vehicle pulls up to roadworthy standards on a road test, the spongy, creeping pedal at idle is not a major issue. It is how the system is and there is not a lot that can be done to change it.