Yeti S+ 2010 2.0TDi CR110 2WD Manual
The detailed symptoms you quote are indeed consistent with fluid leaking back through the master cylinder seals, back to the reservoir. I.E. No external leaks. Pedal initially gives some brake pressure, but slowly sinks to the floor when pressed properly hard with servo (or hydraulic pressure assistance in this case). No loss of fluid level in the reservoir. I initially thought master cyl fault, but ruled that out because you said you had already replaced the master cyl. Perhaps the new cyl is also faulty? Where did you buy it from?The more I think about it the more I feel that the replacement master cylinder is buggy, and that I just can't push it hard enough without the servo to make it give. Must be some way to test it....
But the brakes work great! The ABS works as it should. I just don't like the was it gives.
N.B. In normal circumstances the brake pedal will "feel" halfway normal if you first press it with the engine off. While there is still some residual assistance pressure in the servo or hydraulic pressure reservoir. Two or three presses use that up though, then the pedal feel goes what feels like rock solid. You still have brakes, but you need a LOT of muscle pressure to replicate the assistance level normally provided by the servo/hydraulics. In ABS systems like the Yeti, the assistance pressure is normally provided by a hydraulic pump, rather than a vacuum pressure servo (especially diesels that don't have much manifold vacuum to create the negative side of the servo pressure). In those there is very little reserve assistance. You can hear the ABS hydraulic motor kick in if you listen carefully, as soon as you switch on the ignition.
Oh yes they do! Although "swell" would probably be a better word than "ballooning", which does indeed imply swelling to bursting point. In truth it is the natural but slight and controlled swell in the flexible, rubber hose, sections of the brake lines, that provides the pedal "feel" that you notice when using the brakes. So the pedal does not quite feel like pushing your foot against a rock, but some more progressive pressure, building from moderate to full force. That feel allows drivers with typical skill levels to better modulate the brake pressure according to need. So every gentle push on the brakes does not create the bruising you would when passenger's bodies are crushed against the seat belts, if the pedal was a switch that was fully off or fully on. If you don't believe me, try holding one of the flexible rubber brake lines by gripping it tightly with your fist, while an assistant presses the brake pedal, hard. Preferably with the engine running to provide the pressure assistance. You WILL feel the hose line swell, perhaps by as much as 1mm. Then contract again as the pedal is released. That is normal and designed into the construction of the flexi-hoses.I think it would burst if it started ballooning. There is a LOT of pressure in those lines! The hoses all look good, no sign of cracking or other damage. Checked everything.... dry as a bone.
If you want to experience what the brake pedal would feel like if there were no swell built into the flexi-hoses, try driving a car (such as a rally car), that has had its regular rubber brake hoses replaced by aircraft specification flexible hydraulic lines that have the exterior bore surrounded by stainless steel braiding. Such as those made by Goodridge, or HEL. Some motorcycles use this specification of brake lines too. There is no "give" in the pedal. with or without assistance. Feels like you are pushing your foot on a rock. ALL of the pressure modulation is provided by the driver's foot (left or right depending on driver), and muscle.
Stainless steel brake lines are also much less susceptible to damage caused by flailing rocks within the wheel arch, or a shredded tyre. Kits to replace standard hoses by stainless braided are available for most cars. Just much more expensive. For 98% of circumstances and drivers, the extra cost, coupled with totally different pedal "feel" is just not worth it. I've had fast cars with braided steel brake lines fitted (myself). When you get used to that pedal feel, then jump to a car with regular brake lines it feels initially like you've got no brakes. Much more pedal travel.