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jaguargt

HARD BRAKES - LOW VACUUM? TOO LARGE MC? OR TOO LARGE BOOSTER?

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Hi guys, I'm having a few issues with my brakes: is extermely hard to stop my car, the pedal feels like a rock after about an inch of travel. 

 

I have a 75 280z with Lt1 and a 503 cam, 280zx 10" booster (new), 1" wilwood MC (new), front stock disc brakes and rear drum brakes with all new hardware, and 9 inch of vacuum at the booster port..

 

Do you think the 9 inch of vacuum are too low or could be any other thing to check? 

or maybe having a 10 inch booster and 1 inch MC could be compensated installing toyota 4 pistons caliper?

 

Thank you in advance,

 

Selvin 

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Posted (edited)

9in/hg is probably too low. You need about 12 in/hg minimum. A larger Booster or an Auxiliary Brake Booster Tank will help.

 

You can also increase Idle Manifold vacuum, significantly by Increasing timing advance at idle. Cams with more overlap require more ignition advance at idle. Are you carburated or Fuel injection.

 

You need to run a digressive Vacuum advance curve ( Carb )  or Load curve ( EFI ). With a carb you need a Vacuum advance that has adjustable spring tension and curve . You set the spring tension very weak, mso that full Vacuum advance is all in by 8 to 10 in/hg. Set the stroke length to give you  about 10 degrees of total Vacuum advance. Then connect the Vacuum pot to Manifold Vacuum, so you have full Vacuum at idle. 

 

Set your initial timing to about 14 to 16 degrees BTDC at 700 rpm, with the Vacuum advance hose disconnected. Then connect the Vacuum advance hose to give a total of 24  to 26 degrees . Then  adjust idle to about 1,000 RPM. You should find that this gives a considerable increase in idle Vacuum over Ported Vacuum, or no Vacuum advance at all.

 

The only difference between Ported Vacuum and Manifold Vacuum,  is that Ported source does not pull any Vacuum at idle. Manifold sourced  vacuum does. As soon as you open the throttle Blade slightly, they both behave the same. But the extra 10 degrees of Timing with the Manifold Vacuum really helps cammed engines idle better, use less throttle blade opening to achieve a set idle speed and thus pull more Vacuum at idle.

 

Stand alone EFI is a bit easier to do. You just play with keystrokes and builds the extra timing in by a Programmable curve instead of messing with Vacuum pot spring tensions and stop limiters.

 

Note. It is very important when using a regular distributor with Mechanical weights, that your advance curve does not start  too low. I always set mine so it started around 1,200 rpm with an idle speed of 900 to 1,000 RPM. Having too weak of advance springs will cause the timing to " bounce " at idle. Will cause erratic idle and erratic Vacuum readings. 

Edited by Chickenman

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You didn't mention what Generation of LT1 you have. Early style Gen 1 (1970- 1972 )  or late style Gen 2 ( 1992 - 1997 ) or Gen V ( 2013 - Current ) . The later style have smaller  combustion chambers and require a bit less initial ( mechanical ) timing than the Gen 1.

 

Fuel quality also plays a part of course. So you do have to experiment a bit.

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Thanks a lot for the answer.

 

My engine is a LT1 95, EFI, with stock PCM, I already have Tunner Cat to work on the tunning. I think that I could try advancing the time and I will change the spark plugs too, I was using stock ones and I read that the recommended for the 503 Cam are TR55 NGK. 

 

For now the plan would be:

 

1. Bleed the system, just in case.
2. Re-adjust the rear brakes.
3. Check pedal free travel (rod lenght)
4. Apply vacuum to the booster with an external pump, just to try if there is an improvement, if there is, modify the ignition time with tuner cat. (or maybe a dual diaphragm booster?)
5. Now I think I'm clear that updating to 1" master cylinder without changing the stock components was an error, so, when my budget allow me, for start swap to toyota calipers, I already have the calipers for vented rotors, so I need rebuild the calipers, buy the rotors and spacers. 

Of course, any other ideas are welcome. 

 

Thanks again, 

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On 1/9/2018 at 7:19 AM, jaguargt said:

Hi guys, I'm having a few issues with my brakes: is extermely hard to stop my car, the pedal feels like a rock after about an inch of travel. 

 

Do you mean that the brakes don't slow the car?  Or that the brakes are difficult to control?  The rock-hard pedal is typical of a missing reaction disc in the booster.

 

Measure vacuum at the end of the booster supply hose.  Maybe you have the check valve installed backward.

 

The MC cylinder size won't affect pedal feel as much as pedal travel.

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 You should be able to " Free Rev " the engine a few times and build up the Vacuum in the Booster. On the " Over run " Vacuum will build higher in the Booster. Wing it hard to about 4,000 a few times. Let it return all the way to idle. Repeat about 3 or 4 times. Then press brake pedal. If brake pedal is easier to press, then you know the issue is just low idle Vacuum.

 

If the pedal is still Rock Hard then you may have a problem with the Booster check valve ( Backwards or leaking ) or the reaction disc fell out when you changed the Brake M/Cyl

 

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Thanks good advices, I have chedked the end of the booster supply hose, 9inch of vacuum. I will try with Chickenman advice about revving the engine, good tip. 

 

The booster is new, could be a chance that the reaction disc is out of place, I will check that too.

 

Thanks again, I will keep you posted, 

 

Selvin 

 

 

 

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