The fact that you're actually having to slam the brakes on is kind of worrisome.
Yes, if the fronts lock up first there's potentially a pressure regulator valve issue with the rear, however, testing and setting that valve is a bit tricky.
But aside from that you probably will never be able to fully equalize pressure between the two. If you could it would potentially make stopping in an emergency rather difficult since in that scenario the valve would want to divert from the more powerful front brakes to the rear simply to 'balance' regardless of whether the vehicle would be benefited in stopping by the balancing.
I'm not sure I'm making the point accurately enough so here's another take on it. Balance for the sake of balance is risky when the braking capability is not balanced. I.e. the fronts can do a lot more to stop because of size and because of the load placed on them and I'm not sure that the simple regulator can respond quickly enough to balance this action. And part of that is due to what it uses as a trigger for actuation.
Be that as it may, I'm not even sure the Bentley talks about it very much, so I checked and this is what they say:
"Testing and adjusting the pressure-regulating function requires measuring brake system pressure at each wheel caliper, using two pressure gauges with a range of at least 160 bar (2320 psi). Because of the need for this specialized equipment, we recommend having this test performed by an authorized Volkswagen dealer."
So there you have it, but before verbally calling your pressure regulator defunct you need to perform the test.
Refer to the Bentley for more information on the test.
EDIT: this is one reason we have the advent of ABS systems...
Steve
Only time I'm slamming on the brakes is when I'm trying to test them out - it's not a regular occurrence. The
main concern I have is with stopping in the rain, because there have been a few times that I've had to stop a bit more quickly than usual and the fronts would lock up.
Thanks for the information - at this point I'm mainly just trying to determine if the regular may be a factor and then go from there.
Idunno about newfangled systems with ABS but in the past the fronts were always designed to lock up first. Probably still are in case all the electrickery fails.
The reason being if the fronts lock the car stays relatively straight. If the rears lock first the rear comes around.
Analogy would be balancing a pole on your finger compared to holding the top of it.
Well if that's the case then it may actually be working just as intended.
your tires are garbage
brakes are garbage
i bet there generic junk tires with autozone cheep pads.
Get some Michelins and some bindex pads
the fronts are supposed to lock up before the rears do.
flush the heck out of your brake lines too.
Can't argue with that.
I had already intended to get some better tires next time around. It's too easy to both skid and spin my tires in the rain. Definitely seems like a traction issue.
You can tell all that from Maryland, can you? Your TDI prowess never ceases to amaze me.
Chooch, the proportioning valve isn't that hard to set, usually it's near the neutral mark. I used set them around there and then go for a ride, testing them out. It was never far off the mark.
But I bought a
brake pressure gauge off Amazon and set them with it now. It'll even tell you if you have a line partially collapsed or plugged.
If you want to borrow it, I can toss it in the mail. I don't use it all that much and it just sits. Just make sure the car is level when you do this, with the rear axle beam supported (like its sitting on the tires), otherwise it'll throw off the proportioning valve. While it's not an expensive calibrated gauge, it'll get you good enough to not know the difference.
Hey Abacus, thanks for the offer. I'll let you know if I'm interested later, but I don't think it's all that necessary at the moment.
over the hundreds of cars that have passed though my tools, i have never seen one that had a portioning valve "go bad" or become set wrong. its 1 of 3 things, bad brake fluid, tires, or pads.
OP probably has some locked up rear cylinders and gummed up brake lines. Cars with copper brake lines have a much higher tendency to gum up.
you cant fix a chemical problem with a mechanical fix and vice versa. Fixing or replacing a portioning valve to balance poor brake pad function is probably one of the worst things you can do on a non modified brake set up. Besides OP is saying his fronts lock up before his rears. WELL DUH, there going to do that, a car that is primarily front heavy with a poor suspension geometry is always going to do this. The fact that he is complaining about having to mash the brakes down to get to a stopping clearly shows that his pads and brake fluid are probably to blame if his tires are not poor performing. Its a bit basic.
Quick question though, you put this gauge on the brake caliper bleed screw? how does this work on rears with so little room? an adapter or something? Or do you put it on the rubber line to the caliper?
One thing i have passed by too often is reman brake calipers sticking, every time i get one that isnt working right, i take the piston out to find a few grains of sand or blasting media in there binding the piston to the caliper. Remans are the worst and its very cheep and easy to replace the piston and seals as a DIY and about 2 steps more than a oil change in difficulty.
It is also possible that OP has a leaking rear and brake fluid is saturating the rear shoes.
First thing I did when I bought the car was replace all the calipers with Napa remans. With a lifetime warranty, I've replaced each one of them at least once in the last three years when it started to stick or the piston wouldn't retract properly. Considering the fact that these are free replacements now for as long as I own the car, I'm staying with these because they do actually work. But replacing the pistons/seals is something that I may consider doing later. I've flushed the entire system quite extensively, but it's time to do it again.
As stated on one of my prior replies, yes, it could very potentially be a tire issue. It's pretty easy to spin the tires when the road is wet, and I'm much more likely to lock up the fronts when it's raining. My rear tires have about 10k miles left on them. When they're done I'll put the new tires on the front and go with something better than the Sumic tires I've been running mostly without issue for the last 100,000 miles.
In most of the research I've done it seems a common issue for the proportioning valve lever to freeze up over time and need maintenance. That's why I asked about it, as I didn't know that the fronts were actually
designed to lock up first. I'm not an engineer, and it seemed likely that they should all lock up at approximately the same time.
Yesterday I threw the brakes on around 40 MPH on dry road, and although the fronts did bark a little, the entire stopping power seemed better than it ever has been. Maybe the fresh pads and rotors I put on a few hundred miles ago are starting to break in better now. I have an oil change due soon, at that time I'll actually physically inspect the proportioning valve and report what I find.
EDIT: Also want to add that I don't have to slam/stomp on the brakes for them to operate normally. They certainly don't feel like hydraulic brakes, but have no issue getting me stopped either.
