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braking power unit SP 250

zuurb
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Joined: Tue Mar 29, 2016 8:11 pm

braking power unit SP 250

Post by zuurb »

I have a question directed to owners of a Daimler SP 250 who have equipped their Dart with a vacuum braking power unit. More precisely "where did you position the unit in the engine bay and for what reasons?
Thanks for your response,
Simon

Fossil
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Re: braking power unit SP 250

Post by Fossil »

Hi Simon

There are several options: mine is fitted on the right side of the engine bay, viewed from behind the windscreen, at the front in the corner behind the radiator above the bottom radiator hose. The cyiinder points rearwards, and is tilted slightly upwards. The reason for this odd position is that it came with two fitting brackets to facilitate its installation there, from the supplier David Manners. Generally its position and function have been very successful; it has been there for around 20 years with only one need for intervention by me, which was when I installed a slightly deeper aluminium radiator, requiring the servo brackets to be moved back by about a quarter of an inch. Otherwise it has been completely reliable.

The advantages of this position are that the brake pipe runs in a right hand drive car, from the master cylinder to the servo, and from the servo to the front brake multi-point union, are relatively short, and also that it saves having to go to the trouble of finding somewhere else to fit it, for example in the nose in front of the radiator, which is difficult to access when necessary, or elsewhere in the engine bay where there isn't much space.

I hope that this is helpul.

Fossil

Fossil
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Re: braking power unit SP 250

Post by Fossil »

Simon

Sometimes a picture is worth a thousand words, two are better.

Cheers

Geoff
John Chisholm visit.05.16 011.JPG
John Chisholm visit.05.16 012.JPG

Ozzsp250
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Joined: Tue Feb 16, 2016 12:35 am
Location: Arizona, USA

Re: braking power unit SP 250

Post by Ozzsp250 »

I have the same set up for my brake booster, I do have a question for you - have you experienced similar issues highlighted below?

I have experienced difficulty in obtaining the 'brake pedal' feel I expected with the update. Without the brake booster the pedal is heavy travel is very short brakes work well.

With the booster in the circuit travel is light but pedal travel long and has a spongy feel and it 'pumps up' with a second push on the brake pedal as travel reduces greatly - it appears like air gets trapped in the booster. I have bled multiple times but never improves.

Ian Slade
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Re: braking power unit SP 250

Post by Ian Slade »

With the length of piping bleeding is difficult using the standard method, I used the Gunson bleed kit which pressures the master cylinder ( otther makes are available) this will remove the spongy feeling but the length of travel will be longer due to what is basically two master cylinders in tandem but much lighter pedal
Owner since the 70's, Genghis is slightly to my left.

Ozzsp250
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Location: Arizona, USA

Re: braking power unit SP 250

Post by Ozzsp250 »

Ian, Thanks for the update.

I have tried pressure bleeding as well although the brake booster documentation does not recommend it but no real change noted. It appears air gets stuck in the booster if I temporary bypass it the original travel and feel are restored. I did come across a video where an after market vendor demonstrated bleeding a master cylinder on the work bench before installing it in the car and I may try that. As the car is just a hobby all other things come first I need to complete work on other stuff and then tackle the issue again.

Fossil
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Re: braking power unit SP 250

Post by Fossil »

Dennis

Re the servo related symptoms that you describe I'm not sure that I can help you much. Reference to my service record reveals that I fitted the servo in early 1988, 25,000m ago and I cannot recall any difficulties following its fitting. Yes the pedal was firm with short travel beforehand, and afterwards the travel may have been slightly longer and slightly less firm, but it has never been spongy suggestive of air in the system. There has always been a second push effect of sorts, it is not uncommon in SPs, but the brakes work adequately without the second push, and I only use that if I really want to stop very quickly, although now being an older driver I tend not to encounter such situations very often. For the current obvious reasons it hasn't been on the road this year until a few days ago.

The only problem I've had was in 1999, 10,000m ago, after I'd fitted stainless pistons with new seals to all the calipers, plus braided stainless hoses. Back on the road it was immediately apparent that all the calipers were binding. This was cured by removal of the valve components from the rear caliper restrictor valve, the hollow body being retained, which was the frirst remedy I tried. There have been no problems since. I've used Dot 5 silicone brake fluid continuously since the servo was installed, the last time it was changed was in 99 when that problem was dealt with. Perhaps it's time to add a brake check and rebleed to my to do list. The reservoir hasn't required topping up yet. I've been kept busy doing other major work to it.

Another option to try when bleeding might be a compressed air powered negative pressure venturi bleeder to suck the fluid through.

Sorry not to be more helpful.

Geoff

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Jez
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Re: braking power unit SP 250

Post by Jez »

In response to the OP, my servo is similarly located on the right hand side behind the radiator, although in this case located by a single bracket which is fixed to the curved tie bar on front of the engine. It was installed when we bought the car and has worked without complaint, although the system does have the characteristic soft pedal at the top of the travel on the first push.

I had originally wondered if it was due to pad knock-off and the residual pressure valve not working. A seized rear piston gave me the opportunity to take everything apart and the valve appears to be in good order and having cleaned everything up, filled with silicon fluid and bled through with the pressure bleeder the behaviour was exactly the same. I hadn't thought of it being connected with the servo, so that's worth thinking about, but not sure what else could be done in terms of bleeding. Maybe temporarily adjusting the angle of the servo during bleeding might help??

Cheers - Jez
Jez Stow
1960 Daimler Dart
and a few other toys

tjt77
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Re: braking power unit SP 250

Post by tjt77 »

regarding 'spongy pedal' after fitting a servo/booster :- if the unit is placed high on the car (above height of the master cylinder) bleeding problems are to be expected.. you can use a power bleeder to overcome this (they come with adaptors to fit on the master cylinder and are pressurised by compressed air to push the fluid through the system) Im unsure how much space there is to fit the servo on SP.. on the V8 saloons with std factory servo, its mounted low down, just forward of the right hand front wheel, with a splash guard to keep debris off.. the big healey's have it low down, mounted aft of the r/h front wheel .. The brake pedal will have more movement when an 'inline' servo is fitted regardless.. because the servo needs movement to open the internal valve for vacuum boost..
given the light weight of the SP 250, braking can be improved substantially with a set of 4 piston calipers WITHOUT adding a servo..that way the short pedal travel remains and bleeding is straightforward .. which are reasonably priced today and there are several options.. kits for both XKE and jag Mk2/Daimler V8 saloon are direct bolt in to the SP .. wildwood likely the best option in USA.. Coopercraft in UK.. (the alloy versions by coppercraft or zues all reduce unsprung wieght..which helps ride and handling..std Wilwood kit uses alloy calipers and adapting brackets... coopercraft direct bolt in without adapting brackets )

Ian Slade
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Re: braking power unit SP 250

Post by Ian Slade »

The master brake cylinder is one of highest points under the bonnet of an SP, nothing from Jaguar just bolts onto an SP though some Triumph bits do, please try not to confuse matters.
Owner since the 70's, Genghis is slightly to my left.

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