I own a 1968 XR-7 with power brakes, discs up front and drums in the rear. Im having a lot of trouble with my brake system as I have a soft brake pedal when the car is running but after pumping the pedal stiffens up. Also, while bleeding the pedal is firm.
Anyway ill outline what I’ve done so far. My booster went bad so I had that replaced. In doing so I had to take off the master cylinder to remove and reinstall the booster. After putting the rebuilt booster back on I put the old master on and bled the brakes. After taking it out for a drive I found that the front brakes were sticking and I assumed it was the booster rod sticking too far into the master and was causing them to stick.
So, after taking the master off again, and adjusting the rod back, I put the master back on and bled the brakes (this is gonna be a recurring event throughout this post lol). Once the master was back on and the brakes were bled the pedal was nice and stiff while the car was off but once it was running the pedal went straight to the floor. If it was pumped you could get a pedal but if you let off and waited a couple seconds the pedal would go back to the floor.
Thinking that, I might have ruined the master while bleeding the brakes and pushing gunk through the master I decided I would buy a new master since the old one was dirty and possibly ruined. I order the master (specifically the front discs and rear drums master, dual reservoirs and everything) and wait for it to ship. After it arrives I install the new master (didn’t bench bleed it) and I bleed the brakes and send the fluid through the lines a lot. I mean A LOT in hopes that bleeding the lines over again will bleed the master as well. Wouldn’t you know it, hard pedal while the car is off, right to the floor while the car is running.
Thinking again it might be the booster rod the master comes off and the rod gets adjusted again (almost positive its in the right spot) and the master goes back on and the brakes are bled again. Using the same technique as before, furthest brake to closest to the brake. Again, stiff pedal while off, right to the floor while running.
Im at a loss and I don’t know what else to try. im exploring the potential for the brake distribution block to be bad, but as far as I know a bad block doesn’t have these symptoms. Also, when I was bleeding the brakes I didn’t know about the plunger in the block that sets off the BRAKE warning dash light. Could this be affecting the flow of the brake fluid to the brakes?
Any advice is welcome, thanks for reading the long post and I’m looking forward to hearing some responses. Thanks in advance!
Hmm, for starters what brand and source is the replacement/rebuilt booster? Is it identical part number? Also I wonder if those rod adjustments are meant to be done with brand new pads and fluid topped off? What would cause the front brakes to stick? Could the brake lines to the front and rear reservoirs be swapped on accident?
If you don’t bench bleed the master cylinder it will always have a spongy pedal. Bench bleed it per the instructions in the box. Then install plastic plugs in the ports until the lines can be hooked up. Then bleed the right rear, left rear, right front, and left front in that order. Then repeat in the same order.
You need to adjust the rod length to spec. If it is too short even by a little bit you will have to pump the pedal to get brakes. The spec is in the factory service manual.
I fought a similar problem on my 1967 for years. A bit different in that the pedal never would firm up. I was at the point i was ready to sell and walk away as brakes are a nice thing to have especially with a strong 390GT motor under the bonnet.
Even had one shop basically rebuild the entire system but was honest enough to admit being stumped so charge was minimal.
Finally handed it over to Cougar Bill (Thanks Bill!) who did have to redo a lot of the work but turned out to be the proportioning valve. Bit different for 67 as the valve is separate unit at the rear but a bad or leaking disc brake distribution block/proportioning valve assembly might cause the issues you are seeing.
Also what vacuum are you seeing at the booster? Running a hot cam or anything like that?
The booster is the original midland booster that came off the car. I got it rebuilt. The front brakes were sticking due to the rod being too long which I’ve adjusted back. Ive checked and the front and back lines are going where they’re supposed to be going.
I took the master off to adjust the rod once more. Before it went back onto the car I bench bled it and still having the same result thus far even after bleeding the brakes in the order you mentioned.
I unfortunately don’t have the factory manual, Ive measured the rod and the booster and the rod is 0.16 mm shorter than the depth of the master. I read online that there should be no more than a 0.20 mm cushion so I brought the rod in closer than that. Still to no avail.
Ah I see. I had suspicions that it could be the proportioning valve or the distribution block. I didn’t think the symptoms of the car were consistent with those of a back proportioning valve or bad block. What makes me question this is why did my brakes work fine before I had my booster rebuilt? I hadn’t touched the distribution block at all other than loosening the fittings to the master to get the booster out. Once I reinstalled the rebuilt booster I’ve been facing this problem. The vacuum going to the booster is strong, I haven’t put a gauge to it yet but I had the line off and covered it with my thumb and it nearly sucked the damn thing off lol. Anyway, thank you for your reply, ill look into the valve and block idea!
Hey buddy, its been a while, and I’ve since sold the car. However, if I remember correctly it was another defective master. I assumed since I bought it new, I could rule it out as the culprit, however thats the issue I was chasing the whole time.
Just to throw in some useful info for people searching this thread in the future…if you have a front disc/rear drum brake system make sure the master cylinder you buy has a residual pressure valve in the rear brake circuit. If you don’t have one every time you lift off the brakes the rear shoes will retract, meaning you have to take up that slack again when you depress the pedal. I tore apart my brake system several times until I finally figured out this was the problem. The Dorman (and probably many other) master cylinders listed for a 68 w/ power disc/drum do not have a residual pressure valve. The only one I know for sure that does is the Raybestos model (which unfortunately I believe they have discontinued). Here’s a link to my thread on this with more info and part numbers: Sprucing up a '68 Cougar - #123 by mfc133