1974 Vega, how to convert power brakes to manual?

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1974 Vega, how to convert power brakes to manual?

Postby Bill K » Tue Jul 13, 2010 10:01 pm

I did a search, but didn't really find anything useful for executing this changeover. Do I simply remove the master from the booster, then re-attach the master to the firewall? Do I have to reposition the actuator rod on the brake pedal? Is there anything else involved here? Any advice is welcomed.
Thank you,
Bill
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Re: 1974 Vega, how to convert power brakes to manual?

Postby BadBowtie » Wed Jul 14, 2010 1:56 am

Bill K wrote:I did a search, but didn't really find anything useful for executing this changeover. Do I simply remove the master from the booster, then re-attach the master to the firewall? Do I have to reposition the actuator rod on the brake pedal? Is there anything else involved here? Any advice is welcomed.
Thank you,
Bill


If i'm not mistaken the power master and manual master are different bores. So you would have to remove the booster/master and replace them with a non power master or else you would get a very stiff pedal (or soft, i forget which way it goes). As far as the actuator rod goes, you would have to reposition it, there should be two mounting holes on the brake pedal arm.
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Re: 1974 Vega, how to convert power brakes to manual?

Postby BadBowtie » Wed Jul 14, 2010 2:00 am

P1020013.JPG
brake and clutch pedal
P1020013.JPG (93.1 KiB) Viewed 647 times


The lower pedal is the brake pedal, notice the two positions for the actuator rod.
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1980 Z28 383 stroker T56, 3.42 posi.
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Re: 1974 Vega, how to convert power brakes to manual?

Postby Rickracer » Wed Jul 14, 2010 6:59 am

For manual brakes, you'll want to use the upper position for the brake actuator rod, which will give you more leverage. :th:
Rickracer
ASE Master Auto Technician, 36+ years
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Re: 1974 Vega, how to convert power brakes to manual?

Postby Bill K » Wed Jul 14, 2010 9:21 am

Can I reuse the actuator rod from the power brake booster or do I need to find a rod for a manual brake setup? Thanks in advance, Bill
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Re: 1974 Vega, how to convert power brakes to manual?

Postby 80 MONZTA » Wed Jul 14, 2010 11:53 am

Bill K wrote:Can I reuse the actuator rod from the power brake booster or do I need to find a rod for a manual brake setup? Thanks in advance, Bill

You will need the manual setup, good manual brake rod solution thread here.
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Re: 1974 Vega, how to convert power brakes to manual?

Postby Bill K » Wed Jul 14, 2010 3:15 pm

My car is a Vega but, the brakes both in the front and the rear are from a 1979 Monza (all 4 lug, 13" wheels). Currently the brake pedal is very spongy and the travel of the pedal is excessive. The pedal almost goes to the floor before applying stopping power. They do not give me a confident feeling that the car will stop fast when needed. I purchased the car this way and frankly I hate the pedal feel.
The previous owner built the car this way. I do not know what the master cylinder piston diameter is, It could very well be from a small diameter Vega setup; at this time I do not know. It would make sense that I am not pumping enough fluid to move the caliper pistons and wheel cylinder pistons; this could cause the existing problem. I will gladly keep the power brakes if I can eliminate the pedal sponginess and bring the pedal application point back to within an inch of the normal brake pedal unapplied position. In other words, have the brakes apply with approximately an inch to two inches of pedal travel. The car also has silicone brake fluid, I believe this should not make a difference. Does anyone have any ideas?
Thank you for the very useful link.
Bill
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Re: 1974 Vega, how to convert power brakes to manual?

Postby hammerdown7 » Wed Jul 14, 2010 4:00 pm

Sounds like it needs to be bled, if you have the smaller Vega master that will give you a longer pedal but it shouldn't be mushy.

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Re: 1974 Vega, how to convert power brakes to manual?

Postby cosvega76 » Wed Jul 14, 2010 5:26 pm

Bill K wrote:My car is a Vega but, the brakes both in the front and the rear are from a 1979 Monza (all 4 lug, 13" wheels). Currently the brake pedal is very spongy and the travel of the pedal is excessive. The pedal almost goes to the floor before applying stopping power. They do not give me a confident feeling that the car will stop fast when needed. I purchased the car this way and frankly I hate the pedal feel.
The previous owner built the car this way. I do not know what the master cylinder piston diameter is, It could very well be from a small diameter Vega setup; at this time I do not know. It would make sense that I am not pumping enough fluid to move the caliper pistons and wheel cylinder pistons; this could cause the existing problem. I will gladly keep the power brakes if I can eliminate the pedal sponginess and bring the pedal application point back to within an inch of the normal brake pedal unapplied position. In other words, have the brakes apply with approximately an inch to two inches of pedal travel. The car also has silicone brake fluid, I believe this should not make a difference. Does anyone have any ideas?
Thank you for the very useful link.
Bill



Bill,

Sounds like you have the Vega master (3/4" bore) with the Monza front calipers (2-1/2" pistons). When I first changed over to the S-10 front brakes (2-1/2" piston calipers), I experienced what you are describing. All you need is a Monza master cylinder (7/8" bore) for your power setup and you should be good to go. Measure up your master and see.

And, although it shouldn't be causing your problem in this case, I would flush out your brake system (with denatured alcohol) and replace the silicone fluid with regular DOT 3 or 4 glycol brake fluid. The silicone will give you not as firm of feel, and - the really bad thing - is that the moisture doesn't mix with the fluid so it can corrode the system wherever the droplets of moisture sit. Glycol fluid may mix with the moisture and drop the boiling point, but at least you can flush the old stuff out and renew it. Do NOT mix silicone and glycol fluids - it makes goo.

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Re: 1974 Vega, how to convert power brakes to manual?

Postby Bill K » Wed Jul 14, 2010 8:09 pm

Chuck,
I have owned this car for a couple of years and have barely driven it because of personal constraints and family commitments. The brake system does not have air. I will remove the master and measure the piston Dia. It makes sense that I am displacing all the fluid the small master can supply, stops occur can the pedal is approximately an inch above the floor. It is reasonably solid at that point, but does not provide a secure feeling when driving. I will know more tomorrow.
When changing to the 7/8" master, can I assume the existing booster pushrod will interchange and work(the rod between the booster and master)? I need to take things apart to examine how things are currently assembled. Thanks in advance,
Bill
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Re: 1974 Vega, how to convert power brakes to manual?

Postby cosvega76 » Thu Jul 15, 2010 9:54 am

Bill,

If the master you have is the correct one for power brakes, it should only have a "dimple" in the back of the piston. If this is so, the Monza master will bolt right on. If your master is off of a 1975, you'll have to reverse the brake lines to attach them. If yours if off a '76 or newer, they'll line right up.

Chuck
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Re: 1974 Vega, how to convert power brakes to manual?

Postby Bill K » Thu Jul 15, 2010 2:59 pm

Chuck,
I removed the master and measured the bore, it has a 7/8" dia piston. What I found strange was the following: when I removed the MC cover the front reservoir is the tiny one yet this is where the 1/4" dia steel line is attached. The rear reservoir is considerably larger yet this has only a 3/16" dia steel line attached to the master. In your last post you mentioned about possibly having to reverse the brake lines. Does this make any sense to you? My experience has always been that the disc brake part of the master is the bigger one, the drum brake reservoir is smaller. Incidentially, the lines going to the master are very short, they come directly off the proportioning valve with the brake failure switch.
Bill
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Re: 1974 Vega, how to convert power brakes to manual?

Postby cosvega76 » Thu Jul 15, 2010 6:58 pm

Bill,

That is very curious from the symptoms you describe that you already have a 7/8" bore master. Does your master have adapter fittings to connect to the brake lines? The main reason I was describing reversing the brake lines was because from the factory GM uses fitting sizes of 1/2"-20 and 9/16"-18, and to thread them in you may have to alter their position on the master.

I have heard some folks claim otherwise, but in my experience it makes no difference which line gets attached to which end of the master. In a properly functioning system, pressure should be equal in both ports. And, brakes work off of pressure, not volume, so line size is not critical in the system. That being said, the reason I imagine that OEs use a larger line for the rear brakes may be because of the distance from the master, but I am only speculating.

I would suggest that you make sure all of the air is out of the system, the brake pedal pushrod pin is in the proper location, and all brakes are adjusted properly, to eliminate any excess clearance before making any changes.

Chuck
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Re: 1974 Vega, how to convert power brakes to manual?

Postby Bill K » Thu Jul 15, 2010 8:10 pm

Hi Chuck,
The master has no adapters at the lines to either raise or lower the tubing size. Also, the pushrod from the booster to the brake pedal lever is in the correct hole, thereby lowering the brake pedal closer to the floor. When the engine is not running the pedal is high and firm when applied, the problem arises when the engine is running. When I drove the car today the pedal travel was extreme (almost hits the floor) and the pedal felt mushy. The pedal feel is totally different when the car is running than not running. Could it be I am just accustomed to driving a newer more modern car where the brakes are always firm and right there when applied? I wonder if there is something wrong with my booster? There are no vacuum leaks or engine misses and there is a definite assist when the brakes are pushed. This is why I want to change to a manual brake setup. It is always difficult when a modified car is purchased and you have no idea what the previous owner did to the car when he owned it. The car can become a mystery to figure out.
Bill
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Re: 1974 Vega, how to convert power brakes to manual?

Postby cosvega76 » Sat Jul 17, 2010 7:23 am

Bill,

Do you have rubber brake hoses on the car (as opposed to braided stainless)? Use some clamps made for brake hoses (they have rounded edges), or I have used needle-nose Vise-Grips with short lengths of brake line tubing over the jaws, and clamp off all three hoses at once. Test the pedal with the engine running. If it is high and firm, remove one hose and test. Replace this clamp and remove the next and test. Finally, do the same with the third. This will give you an idea where the problem lies.

You say you have S-10 front brakes on the car. I know of some stock car applications that use the Metric GM calipers bored out for a larger piston. If the calipers on the car have a larger than 2-1/2" piston, you may not enough volume with a 7/8" master. When I upgraded from the single-piston S-10 calipers to the dual 1-1/2" piston Corvette calipers, I ended up with a master with a 15/16" bore.

Chuck
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