Mechanics

BRAKE LINE ROUTE/DIAGRAM

1991 Pontiac Transport • 6 cylinder 2WD Automatic •

I need to know if there is a junction or controller box between my master cylinder and rear brake hoses and drums on my 91 pontiac trans sport - I am thinking there must be a 'T' or something. I'd love a diagram or some images. [There is none in Haines]. Thank you.
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Yeswedeliver
August 25, 2011.



What's the problem or symptom? Do you have anti-lock brakes?

Caradiodoc
Aug 25, 2011.
I just want to visualize before I crawl under the van. I need to know [factory] routing for brake lines - and most important - if and where - there is a junction box between master cyl and wheels[hoses]. It doesn't matter but my peddle went to floor. I found the burst line. It is coroded - the rear one lines all are rotten and I need to replace. I know the line can't simply go to the wheel - there must [?] Be a box, coupler, router, 'T', etc. I need to know where it is and what it looks like. I am disabled and can't get under and out and under and out to examine. I just want to picture what the work is and then get under and do it. I know one needs no diagram to route the lines - just run em where they will go - but it is really the 'box' or 'T' that has me wondering where it is and what it looks like. /Thanks for your help.

Tiny
Yeswedeliver
Aug 25, 2011.
My guess is it doesn't have anti-lock brakes. There will be a separate line going to each wheel. You'll see four steel lines leaving the master cylinder so there's no combination valve or other type of junction block in the system.

Since the pedal went to the floor, there's a good chance the master cylinder is damaged from the lip seals running over the crud and corrosion that build up in the lower halves of the bores where they don't normally travel. There's also a valve built into the master cylinder that blocks two ports when the two hydraulic systems develop unequal pressures as in the case of a leak. This is a split-diagonal system with the left front and right rear on the same hydraulic circuit.

Once you replace the rusted lines, you'll most likely find that no brake fluid will bleed from that rear wheel cylinder and the opposite front caliper. The only way I ever found to reset the valve in the master cylinder is give a short, quick burst of compressed air into one of the opened bleeder screws at a wheel that isn't flowing fluid, then let the line gravity-bleed. If you have to help get the fluid flowing, press the brake pedal no more than half way to the floor to prevent that valve from tripping again and to prevent lip seal damage.

I don't have a diagram to show which brake line goes where. Rather than replacing the entire line all the way to the master cylinder, I find a rust-free section at each end, make double flares there, and install a pre-formed line with the double flares and fittings already on it. If you do just one at a time, you can't mix them up.

Caradiodoc
Aug 25, 2011.
That is helpful, thank you. Yes, [sorry] no anti lock. I am a little unclear on a few things. You refer to a valve and resetting: do you suggest repairing the lines and filling and bleeding as if the valve is OK? Is there a 'test' to see if the valve does need to be reset? Or is that it won't bleed THE test?
One more curiousity - if the system [and I believe you] has this offset - one cylinder to two wheels - why then did the peddle go to the floor and I lost all four? I even closed off the burst line [left rear] - bent and pinched - but I got no peddle or brake but the mechanical saftey at the bottom.
Finally, how much pressure - how hard - to blow and reset the valve is needed? I have no compressor or pressurized tank to give that burst - and this seems stupid - but what would I use for a fitting to seat the air hose to the fitting?
So -if I can't get fluid to those two wheels - I'll have to replace the master cyl - correct?. [If I have no success with resetting with the air pressure].
Ok, you have been very helpful. Just pointing out that there are four lines was a huge help. Had I known - I'd have attemped to patch where it broke.
But why couldn't I get any peddle from the second cyl? Or Line system? When I eyeballed my master cyl - I seem to remember it had only one reservoir - a single - undivided reservoir. I will check again when I go down.
Anyway, you have been great. I have not looked at how the site really works as for paying you or if my donation goes to a general fund. I will ad some dollars though.
Thanks

Tiny
Yeswedeliver
Aug 25, 2011.
The valve is in the master cylinder and might already be tripped. Don't worry about that right now. Get the lines fixed first, fill the reservoir, open the rear bleeder screws, leave the reservoir cap just a little loose so no vacuum builds up that would impede the fluid running down, then take a break until fluid starts dripping. Close the bleeder that's flowing, then wait for the other one to start dripping. If it does, the valve isn't tripped. From then on, never push the brake pedal more than half way to the floor. That should prevent the valve from tripping.

If the second wheel doesn't drip, irritate the brake pedal a little by hand to get the flow started. If the valve is tripped, one rear brake won't have any fluid coming out. You should get a fairly solid pedal, but only two brakes will be working. That IS the test for the valve. No fluid will flow from one rear brake no matter how hard you press the pedal.

As for the no pedal, my guess is both rear lines popped a leak at the same time, or one had popped a while ago and the valve tripped to prevent fluid from going to the leak. You could tell that by excessive wear on the front brake pads on one side and little wear on the other side. The amount of difference depends on how long you would have been driving it like that.

It's also possible the master cylinder got damaged when the leak first occurred. Corrosion builds up in the bottom half of the bores where the lip seals don't normally travel with the pistons. When they finally do go all the way, those seals can be cut by that corrosion. The more common symptom on other brands of cars is the brake pedal will slowly sink to the floor when you hold steady pressure on it. GM master cylinders act differently because they use what's called a "step bore" master cylinder. The piston closest to you is larger in diameter than the second one. It moves more fluid at first to that hydraulic circuit and it pushes some fluid ahead of the front piston, THEN pressure starts to build up. They did that because they used calipers that release more than normal for lower friction and better fuel mileage, but that would leave you with having to move a lot of fluid to apply the calipers. You'd either need a bigger master cylinder or a longer pedal stroke. Either one would require a more aggressive power booster to get the job done. Instead, by using that step bore design, the first part of the pedal movement moves lots of fluid to get the calipers ready to apply, then a little more pedal movement puts the pressure to them.

If the larger seal got damaged, you will have a low pedal because the second smaller piston won't move enough fluid on its own to apply all four brakes. If the smaller secondary piston seal got damaged, the change in the pedal feel is going to be more subtle. GM doesn't seem to have a real lot of trouble with those master cylinders, so we'll cross that bridge later. If that valve is tripped, there's no reason to replace the master cylinder if that's all that's wrong with it. You'll end up with a lot more bleeding and it's entirely possible the valve will trip in the new one too.

On older rear-wheel-drive cars, the hydraulic system was split front and rear. If someone pinched off a leaking line to one front wheel, the car would pull real hard the other way when the brakes were applied. On front-wheel-drive vehicles where a greater percentage of vehicle weight is on the front, having only rear brakes working would cause the rear tires to skid and they'd find you in the next county before the car would stop. By using the split-diagonal system, you'll always have one working front brake when one system has a leak. To prevent that hard pull, they designed in a change to a non-adjustable alignment angle called "scrub radius". Basically what that means to the braking system is the left tire wants to turn to the right from braking forces instead of out to the left. The forces counteract each other instead of adding together so you might not feel any brake pull at all. Chrysler has had that really perfected a long time ago to where people don't even realize there's a braking problem unless the warning light comes on. On other brands, about all you'll see is a little wiggle in the steering wheel if you let go and watch it while braking.

If you have to reset the valve in the master cylinder, I use a rubber-tipped air nozzle to blow air into the opened bleeder screw. To prevent getting carried away, I hold it in position with one hand, then smack and release the handle with my other hand. It just takes a fraction of a second. A little pulse that would push the brake fluid a few inches in the line is sufficient. To try to work the nozzle handle with your thumb would introduce way too much air, and it would take longer than necessary to bleed it out. If you have a hand-operated tire pump with a clamp-on fitting that doesn't screw on, you might be able to make that work. Give the handle one good jab, then take it off and wait for fluid to start running out.

In all areas where you're working with brake fluid, be sure to get no petroleum product of any kind in the fluid. That will really cause an expensive headache. If you need a funnel to fill the master cylinder, it must never have been used for engine oil, power steering fluid, or transmission fluid, unless it was thoroughly washed with brake parts cleaner. Simply wiping it out with a rag isn't good enough. I have to mention that because I read about that here every once in a while.

If a tire pump doesn't work, you might try an air tank from an auto parts store that borrows or rents tools. Get the air nozzle from them too. The advantage with that is you can fill it at a gas station to a lower pressure than air compressors run at. That will help prevent introducing too much air.

Caradiodoc
Aug 25, 2011.