Brake boost repair, BJ8

[peteatgr]

I recently had my '67 freshly painted. It came out beautifully but chose that very moment to develop a brake boost problem. The brakes kept locking up after I had new calipers and brake lines installed. The brake & muffler shop right next to the body shop was sure they could take care of it. Got them the rebuild kit and new leather ring and they tried to rebuild it, unsuccessfully. I now have the vacuum boost unit at home as I try to figure what to do. It has the recently installed rebuild components and new leather ring. It appears the "brake" shop just installed the leather ring on the vacuum piston, rubbed some white grease on the ring and forced the piston into the cylinder. Didn't work for long.

I now have the unit at home with the main body disassembled and have cleaned the cylinder and it looks OK. I'm going to re-coat the cylinder with Sandstrom lubricant. It appears to be in good condition but it is 47 years old so I'm figuring that while it's out is the time to do it no matter how it looks. I'm now starting to shape the piston and ring to fit the cylinder by hand sanding the leather ring to fit the cylinder.

My immediate questions;

How far into the cylinder, while working against the spring, should the piston travel for the boost unit to operate correctly - 1/2 way? 3/4 travel?

Anyone have a better method or tool for shaping the leather ring? I'm hand sanding starting with 60 sandpaper and moving to 150 and 180 to finish the surface.

I've read as many previous threads I could find on BCF and at this point I think this most recent jammed piston problem was just an improperly installed ring from this "repair". But I think something else brought the problem on originally. I've read of people disconnecting the boost and using only the basic manual brake system in the car. How is the unit to be disconnected and still allow relatively normal braking? Just disconnect and cap the vacuum line? Leave the brake fluid lines connected and in place? How do the brakes feel without boost and does it have a big impact on stop distance? It seemed that previous input on this subject was split pretty widely. Would love to hear what others have experienced.

Thanks

 

[Keoke]

Well:
Most generally when the piston jambs on rebuild is caused by the rebuilder not getting and installing a special surface lubricant in side the cylinder it is a spray on material.

 

[peteatgr]

Thanks Keoke. Would you suggest spraying the lubricant before any sanding/shaping of the leather ring?

 

[Keoke]

Thanks Keoke. Would you suggest spraying the lubricant before any sanding/shaping of the leather ring?

I am not sure about sanding the ring.?
Other contributors may have experience with that

 

[BJ8Healeys]

Pete, I don't think you should sand the leather ring at all. In my experience, the problem with sticking of the piston is that the cylindrical foam rubber packing that goes inside the lip of the leather seal to keep it pressed against the cylinder walls was too thick. I had to carefully shave/cut down the diameter of the foam rubber until it fit into the lip without pressing the leather lip too hard against the cylinder wall. The Sandstrom dry film lubricant certainly helps and needs to be applied. I think if everything fits O.K., you should be able to press the piston down into the cylinder as far as it will go and the spring should be able to push it out again. If you push it down and it doesn't come back out, keeping shaving the foam rubber until it does come out, otherwise your brakes will stick on. If they do stick on, sometimes a quick tap on the brake pedal will release them again.

I did not do anything special to disable the booster. It disabled itself because, although it's still connected, after about two years it would not operate. As I mentioned to you before, I believe the rocker valve that ports atmospheric pressure and engine vacuum to the cylinder is not doing its job. Apparently, the engine vacuum port stays open or doesn't seal enough to make the booster work. If you get your piston and rocker valve operating properly, your booster should work.

 

[BJ8Healeys]

Thanks Keoke. Would you suggest spraying the lubricant before any sanding/shaping of the leather ring?

The dry film lubricant should be sprayed on the cylinder walls in accordance with the directions provided and allowed to cure. One type of Sandstrom lubricant requires baking in an oven (and is probably the most durable), but the aerosol spray is air-drying. After it's dry, you can burnish/polish it to make it slicker. The instructions tell you what to do. The leather ring should not be sanded or shaped.

 

[Bob_Spidell]

I rebuilt the booster on my BJ8 and considered it one of the most interesting and rewarding jobs I've done; right up there with rebuilding carburettors and building-up engines. My advice is to read and study the shop manual--you do have a shop manual, right?--and thoroughly understand the theory and function of these units. They are elegant and clever devices (just like SU carburettors). I'm surprised you'd do anything with the leather seal and I don't recall the typical rebuild kits coming with them, but I haven't bought one in a while. Usually, you get the rubber tubing which 'backs up' the leather seal and does need to be trimmed, at least in length.

When I did my booster, I sprayed the Sandstrom dry lubricant although in hindsight it may not have been necessary. I installed the rubber tubing behind the leather seal which AFAIK is original and is now going on 200K miles. What often gives out on these units is the 'gland seal,' which is a small O-ring--I think it had a square cross-section--which seals the vaccuum/air chamber from the hydraulic section. When these leak brake fluid is allowed in the vacuum chamber which subsequently gets sucked into the intake manifold creating an effective smoke generator. The hydraulic section is basically two brake master cylinders; one switches the T-valve Steve mentioned which gates either manifold vacuum or ambient air into the big chamber (note the chamber is under light vacuum when cruising then a lot when you the throttles get closed). When brakes are applied ambient air is vented into the chamber which, given the difference between 15-20" of vacuum--about 10" of air pressure vs. ambient air at 28-29"--and the large surface area of the vacuum piston produces the boosted hydraulic pressure to the drums and/or disks.

Examine the bores of the hydraulic cylinders; if they're scored or pitted get them sleeved. Some say they can be honed, some say no but I say why take a chance. NOTE: If you get the hydraulic section sleeved be sure to tell the shop NOT to media blast the T-switch area; it'll rough up the area and may make a seal impossible. The seals on the vacuum/air T-valve don't (didn't) usually come in the kits; BCS used to sell them and may still. If they aren't damaged or visibly worn they can probably be reused.

Understand what you're doing, take your time and the results should be good.

 

[peteatgr]

Thanks everyone. It's amazing what I sometimes can't see when it's right in front of me.

I've ordered new lubricant spray, and now have a much better understanding of the relationship of the piston, leather ring, and the foam backing piece. The foam needed to be shortened and shaved a bit to allow the piston to release and move freely. The piston was well stuck when I initially opened the chamber. It does take some patience to shave and fit the foam several times to get it right, but a dremel tool with a sanding drum makes the shaving of the foam backing piece pretty easy and allows for good control. The rocker T-seals both look to be in good shape (they don't come with the rebuild kit and neither does the leather ring).

My problem indications were, first, that the brakes would occasionally stick "on" after they had been applied normally. They would always release eventually but the duration of the lock up would vary. I think I understand that this is caused by the piston sticking or not moving smoothly, thus my focus on getting it back to full movement/travel. The second problem indication was a big smoke event at the brake shop just prior to them performing the initial rebuild. The installation of the rebuild kit components by the brake shop seems to have taken care of the smoke problem, but I was still fighting with the problem of the brakes sticking "on". So I am focusing on getting the piston to move and seal properly by restoring the lubricant and by achieving a proper fit of the piston/leather ring/foam backer assembly in the vacuum chamber at this point.

Thanks for everyones advice. And if anyone sees me doing something wrong here please let me know as the brakes have never given me a problem since 1974 so I'm exploring new territory on this.

 

[Bob_Spidell]

Brake sticking can also be caused by collapsed flex lines; with nothing visible externally. If you haven't changed these in a while--certainly if they're original--you should change them since you'll be bleeding the brakes anyway.

 

[peteatgr]

Thanks Bob. I've got all new flexible brake lines and new front calipers on the car and am awaiting delivery of the spray lubricant for the booster piston chamber. Think I've got the piston fit worked out by slowly shaving and fitting the black foam ring backing material. As far as I can see the rest of the booster appears to be in good working condition. I'm hopeful my problem was just a sticking piston and a bad first attempt at rebuild by an inexperienced (AH inexperienced) but well intentioned shop.

 

[Legal Bill]

I sure wish I had my original servo. I have the replacement servo and it just doesn't provide the same level of boost as an original.