On my to-do list this summer was renewal of brake fluid after three years and 6,000 miles. I’ve been using DOT5 for the first time and wasn’t entirely happy with pedal feel so was contemplating a return to DOT3.
The only reason I went to DOT5 is because of the damage DOT3 did to the paint and cad plating on the car prior to its second restoration. At the same time I installed remote rear bleeders, because I dislike the job of taking out the spring/shocks that’re in the way and still skinning my knuckles and getting brake fluid all over. I do like those remote bleeders, but on bleeding the brake system rebuild I was able to achieve only an acceptable pedal feel, not as good as it was with DOT3. A little mushy, made firm by a slight double pump. That double pump said “air bubble” but despite bleeding the brakes a half dozen times more I was unable to improve it. Online posters cited DOT5 compressibilty, which goes against what I learned in liquid physics. DOT5 is not in itself compressible, but it has an unfortunate propensity to retain microbubbles of air. Maybe nanobubbles. Regardless, the brakes worked fine without the little double pump, as a couple of panic stops later demonstrated.
Now, the problem was compounded byt the remote bleeders. They require takeoffs from the tops of calipers that travel downward:
It’s especially a long way down from the left brake caliper but the right’s not much better. Even pushing an air bubble downwards a couple of inches can be a challenge: anybody familiar with bleeding the clutch slave will tell you that.
Back to the summer’s project. Before I got to it I noticed a small wet spot on the garage floor that turned out to be DOT5. One of the aftermarket brake fluid reservoirs had cracked and was more than half empty. The DOT5 was on several surfaces in the engine compartment, including the left subframe, vac servo, mud shields etc. Had it been DOT3 or 4 there would have been considerable cosmetic damage. As it is, none at all.
On cleaning up and putting back my original reservoirs I decided to stick with DOT5, at the same time figuring out a way to bleed the rear brakes in such a way I could be sure I didn’t have any remaining air in the system. I used this to pressurise the front reservoir at 15 psi:
And I made this to put the other end under vacuum at 15 inches of mercury:
With the reservoir under pressure and the bleed jar under vacuum I cracked open the bleed nipple at the remote and the brake fluid gushed forcefully into the jar, taking any possible bubble with it.
I made two lids for the bleed jar. The one resting beside it in the pic has just the one nipple to attach to the vaccuum pump. I 3/4 filled the jar with fresh DOT5 from the FLAPS, screwed on that second lid and put the contents of the jar under 15-18 inches of mercury vacuum. Sure enough, microbubbles came out of the brand new brake fluid, probably just from transporting it home. I did this to the entire litre of new fluid before using it. The brake pedal is now the same as I remember it was using DOT3 and the entire job took no more than ten minutes.
