I’ll try to keep this efficient and fact-based, but there is a lot of info… At the moment I am stumped…
You may recall that two years ago I ditched my Teves III ABS and installed a jag master cylinder, mitsubishi reservoir, and an aftermarket vac booster. (Because I kept the Teves pedal box and the jag booster wouldn’t clear.) I recall a bit of challenge bleeding the brakes at that time, but eventually I used a pressure bleeder and got it done.
Everything was great for almost two years, the car had a good pedal and the brakes worked just fine. About a month ago, the vac booster failed. It was a bit of a shocker when I had “no brakes” but of course what I had was very stiff un-boosted brakes. I drove the car the rest of the way to work and home again at the end of the day, no issues except my leg was tired.
Setting up to replace the vac booster, I had the car in the air and was inspecting everything. I found a leak on the left front caliper. The calipers are original, and since I was gonna have the master cylinder off anyway, I figured no time like the present. I installed reman Raybestos calipers and front brake lines from Rock Auto, and new ceramic pads. Everything fit, no drama.
During this process the front half of the system was completely dry. I flushed out the hard lines with brake cleaner and compressed air since god knows what’s in there after 34 years. I dried them out thoroughly.
I did some research, and instead of purchasing another Summit-branded universal booster, I ordered a universal booster from Master Power Brakes. They claim to be of higher quality. Plus, it’s chrome, so that’s worth 10 hp at least.
Got the new booster in and properly set the clearance to the MC so there is minimal free travel in the pedal before the booster engages the MC. ( This is done via a bolt and locking nut on the front booster rod. Not sure if the Jag booster has this adjustment, but many OEM and aftermarket do. Only mention it in case you are unfamiliar.)
I bought an extra Mitsubishi reservoir cap, drilled it and installed a barb for my Motive pressure bleeder, so I get a good seal with no worries about spraying brake fluid all over.
Assembled, filled, did initial leak check. All good. Using fresh Prestone DOT 3.
Ran a bunch of fluid thru using the pressure bleeder, and finally got a hard pedal. Tiny bit of free travel, then first 1/4 travel is progressively stiffer, then very firm by half way. Seems ok. Started the car to get some vacuum and the pedal is of course much softer, but has same engagement feel. Here’s the rub: If I push pretty hard, I can bottom out the master cylinder. It takes a pretty hard push, but I can do it. I tried with engine off and same thing, but I REALLY have to push to hit bottom. This does not seem OK to me. But I don’t know that I’ve ever tried it before.
On a test drive, it will lock up the rears, but I can’t quite make it lock up the fronts. Not ideal.
I think that on a non-ABS car I should be able to lock up all four wheels if I stomp it hard enough. I did this test after the Teves swap out and could lock up all four, in fact the fronts locked first, which is good. I need to repeat this test to be sure, only did two quick ones from 40 mph.
Here is what I have done:
- Elevate reservoir to make sure nothing is trapped in the feeder hoses to the master cylinder
- Using the pressure bleeder, crack the fittings at the master cylinder to bleed any air that might be right at the MC.
- Repeat above using the press pedal hold crack bleeder method.
- Run several quarts through the system with the pressure bleeder. Not getting any bubbles at any corner.
- Bleed using the conventional method with the bleed tube in a clean container of fluid, a short vertical loop in the tub so the air goes up, press, crack bleeder, hold pedal to floor, close bleeder. Repeat 5 times on each corner. No air.
Here’s the setup. Pressure bleeder is pumped and bleeder is open. No air. Let it run for 3-4 minutes. No air.
The catch can is sitting on this block, so it’s up by the caliper and the bleed hose loop is above the bleed fitting.
- Checked pedal travel for slack in linkage. The front discs start to engage (to the point where I cannot turn the rotor by hand) almost right away, within less than an inch of pedal travel. So my booster to master cylinder adjustment seems good.
- Removed MC from booster, and checked pedal travel. The pedal goes a good inch farther down than with the MC on the booster. I believe this tells me that my linkage is not binding, I am not bottoming in the pedal box or on the floor of the car, I believe I am bottoming the master cylinder.
- Confirmed that front caliper part numbers are the same for ABS and non-ABS cars (getting desperate here…)
- If hold the pedal at the bottom, which takes a whole ton of effort, the force never changes. The master cylinder does not seem to be bleeding down or letting fluid past.
Open to suggestions, I guess my first question is: under extreme braking, is hitting the bottom of the master cylinder normal/acceptable? I would say no. But as I said, I don’t know that I’ve ever actually tried it. Second question, if no, what am I missing in my setup / bleed approach. As I say, the pedal actually feels fine. It just bugs me that if I go ape on it, I can force it to the bottom.
